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Earth-Affecting Coronal Mass Ejections Without Obvious Low Coronal Signatures  

Nariaki Nitta   Submitted: 2017-08-28 10:57

We present a study of the origin of coronal mass ejections (CMEs) that were not accompanied by obvious low coronal signatures (LCSs) and yet were responsible for appreciable disturbances at 1 AU. These CMEs characteristically start slowly. In several examples, extreme ultraviolet (EUV) images taken by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory reveal coronal dimming and a post eruption arcade when we make difference images with long enough temporal separations, which are commensurate with the slow initial development of the CME. Data from the EUV imager and COR coronagraphs of the Sun Earth Connection Coronal and Heliospheric Investigation onboard the Solar Terrestrial Relations Observatory, which provide limb views of Earth-bound CMEs, greatly help us limit the time interval in which the CME forms and undergoes initial acceleration. For other CMEs, we find similar dimming, although only with lower confidence as to its link to the CME. It is noted that even these unclear events result in unambiguous flux rope signatures in in situ data at 1 AU. There is a tendency that the CME source regions are located near coronal holes or open field regions. This may have implications for both the initiation of the stealthy CME in the corona and its outcome in the heliosphere.

Authors: Nariaki V. Nitta, Tamitha Mulligan
Projects: SDO-AIA,STEREO,Wind

Publication Status: Published, Solar Physics, 292, 125 (2017), DOI:10.1007/s11207-017-1147-7
Last Modified: 2017-08-29 09:56
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Solar Sources of 3He-rich Solar Energetic Particle Events in Solar Cycle 24  

Nariaki Nitta   Submitted: 2015-05-26 22:40

Using high-cadence extreme-ultraviolet (EUV) images obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, we investigate the solar sources of 26 3He-rich solar energetic particle (SEP) events at ≲1 MeV nucleon-1 that were well-observed by the Advanced Composition Explorer during solar cycle 24. Identification of the solar sources is based on the association of 3He-rich events with type III radio bursts and electron events as observed by Wind. The source locations are further verified in EUV images from the Solar and Terrestrial Relations Observatory, which provides information on solar activities in the regions not visible from the Earth. Based on AIA observations, 3He-rich events are not only associated with coronal jets as emphasized in solar cycle 23 studies, but also with more spatially extended eruptions. The properties of the 3He-rich events do not appear to be strongly correlated with those of the source regions. As in the previous studies, the magnetic connection between the source region and the observer is not always reproduced adequately by the simple potential field source surface model combined with the Parker spiral. Instead, we find a broad longitudinal distribution of the source regions extending well beyond the west limb, with the longitude deviating significantly from that expected from the observed solar wind speed.

Authors: Nariaki V. Nitta, Glenn M. Mason, Linghua Wang, Christina M. S. Cohen, Mark E. Wiedenbeck
Projects: ACE,SDO-AIA,SoHO-LASCO,STEREO,Wind

Publication Status: Published by the Astrophysical Journal
Last Modified: 2015-06-29 11:56
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The Relation Between Large-Scale Coronal Propagating Fronts and Type II Radio Bursts  

Nariaki Nitta   Submitted: 2014-09-22 13:43

Large-scale, wave-like disturbances in extreme-ultraviolet (EUV) and type II radio bursts are often associated with coronal mass ejections (CMEs). Both phenomena may signify shock waves driven by CMEs. Taking EUV full-disk images at an unprecedented cadence, the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory has observed the so-called EIT waves or large-scale coronal propagating fronts (LCPFs) from their early evolution, which coincides with the period when most metric type II bursts occur. This article discusses the relation of LCPFs as captured by AIA with metric type II bursts. % which may not be simple. We show examples of type II bursts without a clear LCPF and fast LCPFs without a type II burst. Part of the disconnect between the two phenomena may be due to the difficulty in identifying them objectively. Furthermore, it is possible that the individual LCPFs and type II bursts may reflect different physical processes and external factors. In particular, the type II bursts that start at low frequencies and high altitudes tend to accompany an extended arc-shaped feature, which probably represents the 3D structure of the CME and the shock wave around it, rather than its near-surface track, which has usually been identified with EIT waves. This feature expands and propagates toward and beyond the limb. These events may be characterized by stretching of field lines in the radial direction, and be distinct from other LCPFs, which may be explained in terms of sudden lateral expansion of the coronal volume. Neither LCPFs nor type II bursts by themselves serve as necessary conditions for coronal shock waves, but these phenomena may provide useful information on the early evolution of the shock waves in 3D when both are clearly identified in eruptive events.

Authors: Nariaki V. Nitta, Wei Liu, Nat Gopalswamy, Seiji Yashiro
Projects: SDO-AIA

Publication Status: Solar Physics, in press
Last Modified: 2014-09-22 21:07
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The Relation Between Large-Scale Coronal Propagating Fronts and Type II Radio Bursts  

Nariaki Nitta   Submitted: 2014-09-22 13:43

Large-scale, wave-like disturbances in extreme-ultraviolet (EUV) and type II radio bursts are often associated with coronal mass ejections (CMEs). Both phenomena may signify shock waves driven by CMEs. Taking EUV full-disk images at an unprecedented cadence, the {it Atmospheric Imaging Assembly} (AIA) onboard the Solar Dynamics Observatory has observed the so-called EIT waves or large-scale coronal propagating fronts (LCPFs) from their early evolution, which coincides with the period when most metric type II bursts occur. This article discusses the relation of LCPFs as captured by AIA with metric type II bursts. % which may not be simple. We show examples of type II bursts without a clear LCPF and fast LCPFs without a type II burst. Part of the disconnect between the two phenomena may be due to the difficulty in identifying them objectively. Furthermore, it is possible that the individual LCPFs and type II bursts may reflect different physical processes and external factors. In particular, the type II bursts that start at low frequencies and high altitudes tend to accompany an extended arc-shaped feature, which probably represents the 3D structure of the CME and the shock wave around it, rather than its near-surface track, which has usually been identified with EIT waves. This feature expands and propagates toward and beyond the limb. These events may be characterized by stretching of field lines in the radial direction, and be distinct from other LCPFs, which may be explained in terms of sudden lateral expansion of the coronal volume. Neither LCPFs nor type II bursts by themselves serve as necessary conditions for coronal shock waves, but these phenomena may provide useful information on the early evolution of the shock waves in 3D when both are clearly identified in eruptive events.

Authors: Nariaki V. Nitta, Wei Liu, Nat Gopalswamy, Seiji Yashiro
Projects: SDO-AIA

Publication Status: Solar Physics, now published online
Last Modified: 2014-10-14 09:58
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Solar Cycle Variations of the Radio Brightness of the Solar Polar Regions as Observed by the Nobeyama Radioheliograph  

Nariaki Nitta   Submitted: 2013-12-18 17:36

We have analyzed daily microwave images of the Sun at 17 GHz obtained with the Nobeyama Radioheliograph (NoRH) in order to study the solar cycle variations of the enhanced brightness in the polar regions. Unlike in previous works, the averaged brightness of the polar regions is obtained from individual images rather than from synoptic maps. We confirm that the brightness is anti-correlated with the solar cycle and that it has generally declined since solar cycle 22. Including images up to 2013 October, we find that the 17 GHz brightness temperature of the south polar region has decreased noticeably since 2012. This coincides with a significant decrease in the average magnetic field strength around the south pole, signaling the arrival of solar maximum conditions in the southern hemisphere more than a year after the northern hemisphere. We do not attribute the enhanced brightness of the polar regions at 17 GHz to the bright compact sources that occasionally appear in synthesized NoRH images. This is because they have no correspondence with small-scale bright regions in images from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory with a broad temperature coverage. Higher-quality radio images are needed to understand the relationship between microwave brightness and magnetic field strength in the polar regions.

Authors: Nariaki V. Nitta, Xudong Sun, J. Todd Hoeksema, Marc L. DeRosa
Projects: Nobeyama Radioheliograph,SDO-AIA

Publication Status: Published, 2014, ApJL, 780, L23
Last Modified: 2013-12-19 07:43
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Large-scale Coronal Propagating Fronts in Solar Eruptions as Observed by the Atmospheric Imaging Assembly on Board the Solar Dynamics Observatory - An Ensemble Study  

Nariaki Nitta   Submitted: 2013-08-18 21:49

This paper presents a study of a large sample of global disturbances in the solar corona with characteristic propagating fronts as intensity enhancement, similar to the phenomena that have often been referred to as EIT waves or EUV waves. Now Extreme Ultraviolet (EUV) images obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) provide a significantly improved view of these large-scale coronal propagating fronts (LCPFs). Between April 2010 and January 2013, a total of 171 LCPFs have been identified through visual inspection of AIA images in the 193 Å channel. Here we focus on the 138 LCPFs that are seen to propagate across the solar disk, first studying how they are associated with flares, coronal mass ejections (CMEs) and type II radio bursts. We measure the speed of the LCPF in various directions until it is clearly altered by active regions or coronal holes. The highest speed is extracted for each LCPF. It is often considerably higher than EIT waves. We do not find a pattern where faster LCPFs decelerate and slow LCPFs accelerate. Furthermore, the speeds are not strongly correlated with the flare intensity or CME magnitude, nor do they show an association with type II bursts. We do not find a good correlation either between the speeds of LCPFs and CMEs in a subset of 86 LCPFs observed by one or both of the Solar and Terrestrial Relations Observatory (STEREO) spacecraft as limb events.

Authors: Nitta, N. V., Schrijver, C. J., Title, A. M., Liu, W.
Projects: SDO-AIA

Publication Status: Published, ApJ, 776, 58 (2013)
Last Modified: 2013-09-30 10:01
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Large-scale Coronal Propagating Fronts in Solar Eruptions as Observed by the Atmospheric Imaging Assembly on Board the Solar Dynamics Observatory - An Ensemble Study  

Nariaki Nitta   Submitted: 2013-08-18 21:49

This paper presents a study of a large sample of global disturbances in the solar corona with characteristic propagating fronts as intensity enhancement, similar to the phenomena that have often been referred to as EIT waves or EUV waves. Now Extreme Ultraviolet (EUV) images obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) provide a significantly improved view of these large-scale coronal propagating fronts (LCPFs). Between April 2010 and January 2013, a total of 171 LCPFs have been identified through visual inspection of AIA images in the 193 Å channel. Here we focus on the 138 LCPFs that are seen to propagate across the solar disk, first studying how they are associated with flares, coronal mass ejections (CMEs) and type II radio bursts. We measure the speed of the LCPF in various directions until it is clearly altered by active regions or coronal holes. The highest speed is extracted for each LCPF. It is often considerably higher than EIT waves. We do not find a pattern where faster LCPFs decelerate and slow LCPFs accelerate. Furthermore, the speeds are not strongly correlated with the flare intensity or CME magnitude, nor do they show an association with type II bursts. We do not find a good correlation either between the speeds of LCPFs and CMEs in a subset of 86 LCPFs observed by one or both of the Solar and Terrestrial Relations Observatory (STEREO) spacecraft as limb events.

Authors: Nitta, N. V., Schrijver, C. J., Title, A. M., Liu, W.
Projects: SDO-AIA

Publication Status: Published, 2013, ApJ, 776, 58
Last Modified: 2014-09-22 21:07
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Large-scale Coronal Propagating Fronts in Solar Eruptions as Observed by the Atmospheric Imaging Assembly on Board the Solar Dynamics Observatory - An Ensemble Study  

Nariaki Nitta   Submitted: 2013-08-18 21:49

This paper presents a study of a large sample of global disturbances in the solar corona with characteristic propagating fronts as intensity enhancement, similar to the phenomena that have often been referred to as EIT waves or EUV waves. Now Extreme Ultraviolet (EUV) images obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) provide a significantly improved view of these large-scale coronal propagating fronts (LCPFs). Between April 2010 and January 2013, a total of 171 LCPFs have been identified through visual inspection of AIA images in the 193 Å channel. Here we focus on the 138 LCPFs that are seen to propagate across the solar disk, first studying how they are associated with flares, coronal mass ejections (CMEs) and type II radio bursts. We measure the speed of the LCPF in various directions until it is clearly altered by active regions or coronal holes. The highest speed is extracted for each LCPF. It is often considerably higher than EIT waves. We do not find a pattern where faster LCPFs decelerate and slow LCPFs accelerate. Furthermore, the speeds are not strongly correlated with the flare intensity or CME magnitude, nor do they show an association with type II bursts. We do not find a good correlation either between the speeds of LCPFs and CMEs in a subset of 86 LCPFs observed by one or both of the Solar and Terrestrial Relations Observatory (STEREO) spacecraft as limb events.

Authors: Nitta, N. V., Schrijver, C. J., Title, A. M., Liu, W.
Projects: SDO-AIA

Publication Status: Published as ApJ, 776, 58 (2013)
Last Modified: 2015-01-04 10:55
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The Association of Solar Flares with Coronal Mass Ejections During the Extended Solar Minimum  

Nariaki Nitta   Submitted: 2013-08-18 21:44

We study the association of solar flares with coronal mass ejections (CMEs) during the deep, extended solar minimum of 2007-2009, using extreme-ultraviolet (EUV) and white-light (coronagraph) images from the Solar Terrestrial Relations Observatory (STEREO). Although all of the fast (v > 900 km s-1) and wide (theta > 100 deg) CMEs are associated with a flare that is at least identified in GOES soft X-ray light curves, a majority of flares with relatively high X-ray intensity for the deep solar minimum (e.g. >=1 10-6 W m-2 or C1) are not associated with CMEs. Intense flares tend to occur in active regions with strong and complex photospheric magnetic field, but the active regions that produce CME-associated flares tend to be small, including those that have no sunspots and therefore no NOAA active-region numbers. Other factors on scales comparable to and larger than active regions seem to exist that contribute to the association of flares with CMEs. We find the possible low coronal signatures of CMEs, namely eruptions, dimmings, EUV waves, and Type III bursts, in 91%, 74%, 57%, and 74%, respectively, of the 35 flares that we associate with CMEs. None of these observables can fully replace direct observations of CMEs by coronagraphs.

Authors: Nitta, N. V., Aschwanden, A. M., Freeland, S. L., Lemen, J. R., Wuelser, J.-P., Zarro, D. M.
Projects: STEREO

Publication Status: Published as Solar Phys., 289, 1257 (2014)
Last Modified: 2015-01-04 10:57
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Observables Indicating Two Major Coronal Mass Ejections During the WHI  

Nariaki Nitta   Submitted: 2013-08-18 21:36

Two of the five fast (v >= 900 km s-1) coronal mass ejections (CMEs) between January 2007 and December 2009 were observed during the Whole Heliosphere Interval (WHI: 20 March ? 16 April 2008). The main purpose of this article is to discuss possible observational signatures that could have been used to predict these CMEs. During the WHI, there were three active regions aligned almost East?West in a longitudinal span of about 60 degrees. They were NOAA active region (AR) 10987, 10988, and 10989. In terms of the sunspot area, AR 10988 was the largest. However, the fast CMEs were launched from AR 10989 on 25 March and from AR 10987 on 5 April. One explanation for this may be that AR 10988, unlike the other two regions, emerged underneath a predominantly closed magnetic-field environment, as shown by global magnetic-field extrapolations. Around the times of these CMEs, however, magnetic-field observations of the source regions were essentially missing, because they were close to, or behind, the limb as viewed from Earth. Therefore, we explore an extended view in longitude of the regions from the Solar Terrestrial Relations Observatory (STEREO). The two STEREO spacecraft were located ~24 degrees East and West of the Sun?Earth line during the period of interest. We study the frequency of microflares in the three regions and changes in large-scale structures including streamers, but the CMEs do not seem to be correlated with either of them. Instead, activation of filaments or prominences may directly signal subsequent eruptions.

Authors: N.V. Nitta
Projects: STEREO

Publication Status: Published, Solar Physics, 274, 219, 2011
Last Modified: 2013-08-20 07:02
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What Are Special About Ground-Level Events?  

Nariaki Nitta   Submitted: 2012-05-24 17:34

Ground level events (GLEs) occupy the high-energy end of gradual solar energetic particle (SEP) events. They are associated with coronal mass ejections (CMEs) and solar flares, but we still do not clearly understand the special conditions that produce these rare events. During Solar Cycle 23, a total of 16 GLEs were registered by ground-based neutron monitors. We first ask if these GLEs are clearly distinguishable from other SEP events observed from space. Setting aside possible difficulties in identifying all GLEs consistently, we then try to find observables which may unmistakably isolate these GLEs by studying the basic properties of the associated eruptions and the active regions (ARs) that produced them. It is found that neither the magnitudes of the CMEs and flares nor the complexities of the ARs give sufficient conditions for GLEs. It is possible to find CMEs, flares or ARs that are not associated with GLEs but that have more extreme properties than those associated with GLEs. We also try to evaluate the importance of magnetic field connection of the AR with Earth on the detection of GLEs and their onset times. Using the potential field source surface (PFSS) model, a half of the GLEs are found to be well-connected. However, the GLE onset time with respect to the onset of the associated flare and CME does not strongly depend on how well-connected the AR is. The GLE onset behavior may be largely determined by when and where the CME-driven shock develops. We could not relate the shocks responsible for the onsets of past GLEs with features in solar images, but the combined data from the Solar TErrestrial RElations Observatory (STEREO) and the Solar Dynamics Observatory (SDO) have the potential to change this for GLEs that may occur in the rising phase of Solar Cycle 24.

Authors: N.V. Nitta, Y. Liu, M.L. DeRosa, R.W. Nightingale
Projects: GOES Particles,GOES X-rays ,Neutron Monitors,Other,SoHO-EIT,SoHO-MDI,SoHO-LASCO,Wind

Publication Status: Published as Space Sci. Rev., 171, 61 (2012)
Last Modified: 2015-01-04 11:00
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An Alternative View of the ''Masuda''' Flare  

Nariaki Nitta   Submitted: 2010-12-29 00:49

The limb flare on 1992 January 13, the so-called Masuda flare, has stimulated scientists to refine theory of solar flares based on two-dimensional magnetic reconnection. This is primarily because of the hard X-ray source seen above the clearly defined flare loop, and the outward motions in soft X-rays interpreted as ''plasmoid'' ejections. We have revisited Yohkoh hard and soft X-ray data for this and other limb flares, and found that the Masuda flare is still unique in terms of the location and spectral properties of the coronal hard X-ray source. However, the outward motions outside the flare loop may not be as simply characterized as plasmoid ejections as in other flares, nor are they particularly fast. The motions appear complex partly because we also see trans-equatorial loops in motion, one of whose legs anchor close to the main flare loop. It is possible that these large-scale loops represent post-flare loops, and that the flare may also be explained in terms of three-dimensional quadrupolar reconnection, similar to those flares where a pair of two loops exchange their foot-points through magnetic reconnection. It appears that expansion and brightening of large-scale loops offset from the main flare loop are not common, possibly providing a reason for the unusual coronal hard X-ray source in the Masuda flare.

Authors: Nariaki V. Nitta, Samuel L. Freeland, Wei Liu
Projects: Yohkoh-HXT

Publication Status: Published. ApJ Letters, 725, L28 (2010).
Last Modified: 2010-12-29 10:03
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Subject will be restored when possible  

Nariaki Nitta   Submitted: 2008-02-03 14:03

We study the solar source of the 3He-rich solar energetic particle (SEP)event observed on 2006 November 18. The SEP event showed a clearvelocity dispersion at energies below 1 MeV nucleon-1, indicating itssolar origin. We associate the SEP event with a coronal jet in anactive regionat heliographic longitude of W50, as observed in soft X-rays. This jetwas the only noticeable activity in full-disk X-ray images around theestimated release time of the ions. It was temporally correlated with aseries of type III radio bursts detected in metric and longer wavelengthranges, and was followed by a non-relativistic electron event. The jetmay be explained in terms of the model of an expanding loop reconnectingwith large-scale magnetic field, which is open to interplanetary spacefor the particles to be observed at 1 AU. The open field lines appear to beanchored at the boundary between the umbra and penumbra of the leadingsunspot, where a brightening is observed in both soft and hard X-raysduring the jet activity. Other flares in the same region possiblyassociated with 3He-rich SEP events were not accompanied by a jet,indicative of different origins of this type of SEP events.

Authors: Nariaki V. Nitta, Glenn M. Mason, Mark E. Wiedenbeck, Christina M. S. Cohen, Säm Krucker, Iain G. Hannah, Masumi Shimojo, and Kazunari Shibata
Projects:

Publication Status: Will appear in ApJ Letters (10-Mar-2008)
Last Modified: 2008-02-03 14:03
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Subject will be restored when possible  

Nariaki Nitta   Submitted: 2008-02-03 14:03

We study the solar source of the 3He-rich solar energetic particle (SEP) event observed on 2006 November 18. The SEP event showed a clear velocity dispersion at energies below 1 MeV nucleon-1, indicating its solar origin. We associate the SEP event with a coronal jet in an active region at heliographic longitude of W50, as observed in soft X-rays. This jet was the only noticeable activity in full-disk X-ray images around the estimated release time of the ions. It was temporally correlated with a series of type III radio bursts detected in metric and longer wavelength ranges, and was followed by a non-relativistic electron event. The jet may be explained in terms of the model of an expanding loop reconnecting with large-scale magnetic field, which is open to interplanetary space for the particles to be observed at 1 AU. The open field lines appear to be anchored at the boundary between the umbra and penumbra of the leading sunspot, where a brightening is observed in both soft and hard X-rays during the jet activity. Other flares in the same region possibly associated with 3He-rich SEP events were not accompanied by a jet, indicative of different origins of this type of SEP events.

Authors: Nariaki V. Nitta, Glenn M. Mason, Mark E. Wiedenbeck, Christina M. S. Cohen, Säm Krucker, Iain G. Hannah, Masumi Shimojo, and Kazunari Shibata
Projects: None

Publication Status: Published in ApJL (10-Mar-2008)
Last Modified: 2008-02-28 12:37
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Subject will be restored when possible  

Nariaki Nitta   Submitted: 2008-02-03 14:03

We study the solar source of the 3He-rich solar energetic particle (SEP) event observed on 2006 November 18. The SEP event showed a clear velocity dispersion at energies below 1 MeV nucleon-1, indicating its solar origin. We associate the SEP event with a coronal jet in an active region at heliographic longitude of W50, as observed in soft X-rays. This jet was the only noticeable activity in full-disk X-ray images around the estimated release time of the ions. It was temporally correlated with a series of type III radio bursts detected in metric and longer wavelength ranges, and was followed by a non-relativistic electron event. The jet may be explained in terms of the model of an expanding loop reconnecting with large-scale magnetic field, which is open to interplanetary space for the particles to be observed at 1 AU. The open field lines appear to be anchored at the boundary between the umbra and penumbra of the leading sunspot, where a brightening is observed in both soft and hard X-rays during the jet activity. Other flares in the same region possibly associated with 3He-rich SEP events were not accompanied by a jet, indicative of different origins of this type of SEP events.

Authors: Nariaki V. Nitta, Glenn M. Mason, Mark E. Wiedenbeck, Christina M. S. Cohen, Säm Krucker, Iain G. Hannah, Masumi Shimojo, and Kazunari Shibata
Projects:

Publication Status: Published (2008, ApJL, 675, L125)
Last Modified: 2008-03-08 16:44
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Subject will be restored when possible  

Nariaki Nitta   Submitted: 2008-02-01 23:16

For heliophysics research and applications, the potential field source surface (PFSS) model is often employed to extrapolate the photospheric magnetic field to the corona. In an attempt to evaluate the performance of the PFSS model, we compare the computed foot-points of the heliospheric magnetic field with the locations of flares associated with type III radio bursts, which are a good indicator of open field lines that extend to interplanetary space. Consistent with past experiences, the agreement is not satisfactory. We discuss possible reasons for the discrepancy, including the model's inadequacy to reproduce the coronal magnetic field above evolving active regions and the lack of a simultaneous full-surface magnetic map. It is argued that the performance of the PFSS model needs to be quantified further against solar observations including type III bursts before it is applied to heliospheric models.

Authors: Nariaki V. Nitta and Marc L. DeRosa
Projects: None

Publication Status: Published, 2008, ApJ Letters, 673, L207
Last Modified: 2008-02-03 09:23
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Subject will be restored when possible  

Nariaki Nitta   Submitted: 2008-02-01 21:14

For heliophysics research and applications, the potential field source surface (PFSS) model is often employed to extrapolate the photospheric magnetic field to the corona. In an attempt to evaluate the performance of the PFSS model, we compare the computed foot-points of the heliospheric magnetic field with the locations of flares associated with type III radio bursts, which are a good indicator of open field lines that extend to interplanetary space. Consistent with past experiences, the agreement is not satisfactory. We discuss possible reasons for the discrepancy, including the model's inadequacy to reproduce the coronal magnetic field above evolving active regions and the lack of a simultaneous full-surface magnetic map. It is argued that the performance of the PFSS model needs to be quantified further against solar observations including type III bursts before it is applied to heliospheric models.

Authors: Nariaki V. Nitta and Marc L. DeRosa
Projects: None

Publication Status: Published, 2008, ApJ Letters, 673, L207
Last Modified: 2008-02-01 21:14
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Solar Sources of Impulsive Solar Energetic Particle Events and Their Magnetic Field Connection to the Earth  

Nariaki Nitta   Submitted: 2006-07-11 08:21

This paper investigates the solar origin of impulsive solar energetic particle (SEP) events, often referred to as 3He-rich flares, by attempting to locate the source regions of 117 events as observed at ~2-3 MeV/amu. Given large uncertainties as to when ions at these energies were injected, we use type III radio bursts that occur within a 5-hour time window preceding the observed ion onset, and search in EUV and X-ray full-disk images for brightenings around the times of the type III bursts. In this way we find the solar sources in 69 events. High cadence EUV images often reveal a jet in the source region shortly after the type III burst. We also study magnetic field connections between the Earth and the solar sources of impulsive SEP events as identified above, combining the potential field source surface (PFSS) model for the coronal field and the Parker spiral for the interplanetary magnetic field. We find open field lines in and around ~80% of the source regions. But only in ~40% of the cases, can we find field lines that are both close to the source region at the photosphere and to the Parker spiral coordinates at the source surface, suggesting challenges in understanding the Sun-Earth magnetic field with observations available presently and in near future.

Authors: Nariaki V. Nitta, Donald V. Reames, Marc L. DeRosa, Yang Liu, Seiji Yashiro, and Natchimuthuk Gopalswamy
Projects:

Publication Status: Published (2006, ApJ, 650, 438)
Last Modified: 2006-10-23 16:12
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Low Coronal Signatures of Large Solar Energetic Particle Events  

Nariaki Nitta   Submitted: 2003-02-06 14:20

We report on the low coronal signatures of major solar energetic particle (SEP) events. Because large SEP events are highly associated with both flares and coronal mass ejections (CMEs), we focused on flare-associated motions in soft X-rays. In a sample of a half dozen well-observed flares associated with SEP events, we identified two basic types of motions or ejections. For one class of events including those of 2001 November 4 and 1998 April 20, the ejections occur on active region or larger scales. They have an extended "pre-eruption" phase in which the involved structures slowly rise or expand on time scales of tens of of minutes. For the second class of events, including those on 1997 November 6 and 2001 April 15, the large-scale pre-eruption phase is absent. In these events, ejecta appear explosively at the onset of the flare impulsive phase. The observed differences in ejections appear to correlate with spectral/compositional/charge state characteristics of large SEP events, suggesting that flare ejecta are diagnostic of shock properties/environment.

Authors: Nariaki V. Nitta, Edward W. Cliver, and Allan J. Tylka
Projects:

Publication Status: in press ApJL
Last Modified: 2003-02-06 14:20
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Recurrent flare/CME events from an emerging flux region  

Nariaki Nitta   Submitted: 2001-08-28 12:35

We report on six recurrent `halo' coronal mass ejections (CMEs) that occurred (in November 2000) during a 60-hour period in clear association with major flares in an active region on the solar disk. The region was undergoing dynamic restructuring due to flux emergence. The flares were not long-decay events (LDEs) in terms of soft X-ray light curves and morphologies, although, in the impulsive phase, they produced ejections in soft X-rays that are characteristic of CMEs. We do not detect global changes in EUV and X-ray full-disk images prior to these flares. We suggest that emerging magnetic flux in the core of an active region may be responsible for the occurrence of such repeated flare/CME events.

Authors: Nitta, N. V., Hudson, H. S.
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Publication Status: GRL (in press)
Last Modified: 2001-08-28 12:35
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