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Anomalous Expansion of Coronal Mass Ejections during Solar Cycle 24 and its Space Weather Implications  

Nat Gopalswamy   Submitted: 2014-04-01 18:47

The familiar correlation between the speed and angular width of coronal mass ejections (CMEs) is also found in solar cycle 24, but the regression line has a larger slope: for a given CME speed, cycle 24 CMEs are significantly wider than those in cycle 23. The slope change indicates a significant change in the physical state of the heliosphere, due to the weak solar activity. The total pressure in the heliosphere (magnetic + plasma) is reduced by ~40%, which leads to the anomalous expansion of CMEs explaining the increased slope. The excess CME expansion contributes to the diminished effectiveness of CMEs in producing magnetic storms during cycle 24, both because the magnetic content of the CMEs is diluted and also because of the weaker ambient fields. The reduced magnetic field in the heliosphere may contribute to the lack of solar energetic particles accelerated to very high energies during this cycle.

Authors: Nat Gopalswamy, Sachiko Akiyama, Seiji Yashiro, Hong Xie, and Pertti M?kel?, Grzegorz Michalek
Projects: GOES X-rays ,Nobeyama Radioheliograph,SDO-AIA,SoHO-EIT,SoHO-LASCO,STEREO

Publication Status: Accepted for publication in GRL, April 1, 2014
Last Modified: 2014-04-02 08:46
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Obscuration of Flare Emission by an Eruptive Prominence  

Nat Gopalswamy   Submitted: 2013-09-08 22:59

We report on the eclipsing of microwave flare emission by an eruptive prominence from a neighboring region as observed by the Nobeyama Radioheliograph at 17 GHz. The obscuration of the flare emission appears as a dimming feature in the microwave flare light curve. We use the dimming feature to derive the temperature of the prominence and the distribution of heating along the length of the filament. We find that the prominence is heated to a temperature above the quiet Sun temperature at 17 GHz. The duration of the dimming is the time taken by the eruptive prominence in passing over the flaring region. We also find evidence for the obscuration in EUV images obtained by the Solar and Heliospheric Observatory (SOHO) mission.

Authors: Nat Gopalswamy ans seiji Yashiro
Projects: GOES X-rays ,Nobeyama Radioheliograph,SoHO-EIT,SoHO-LASCO

Publication Status: Accepted for Publication in PASJ 2013 September 3
Last Modified: 2013-09-09 10:12
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Topical Issue in Solar Physics: Flux-rope Structure of Coronal Mass Ejections: Preface  

Nat Gopalswamy   Submitted: 2013-03-30 05:12

This Topical Issue of Solar Physics, devoted to the study of flux-rope structure in coronal mass ejections (CMEs), is based on two Coordinated Data Analysis Workshops (CDAWs) held in 2010 (20 - 23 September in Dan Diego, California, USA) and 2011 (September 5-9 in Alcala, Spain). The primary purpose of the CDAWs was to address the question: Do all CMEs have flux rope structure? There are 18 papers om this topical issue, including this preface.

Authors: N. Gopalswamy, T. Nieves-Chinchilla, M. Hidalgo, J. Zhang, P. Riley, and L. van Driel- Gesztelyi, C.H. Mandrini
Projects: None,ACE,GOES X-rays ,Nobeyama Radioheliograph,SoHO-EIT,SoHO-MDI,SoHO-LASCO,Wind,Yohkoh-SXT

Publication Status: Solar Phys. in press, doi: 10.1007/s11207-013-0280-1
Last Modified: 2013-04-01 10:29
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The First Ground Level Enhancement Event of Solar Cycle 24: Direct Observation of Shock Formation and Particle Release Heights  

Nat Gopalswamy   Submitted: 2013-02-07 00:09

We report on the 2012 May 17 Ground Level Enhancement (GLE) event, which is the first of its kind in Solar Cycle 24. This is the first GLE event to be fully observed close to the surface by the Solar Terrestrial Relations Observatory (STEREO) mission. We determine the coronal mass ejection (CME) height at the start of the associated metric type II radio burst (i.e., shock formation height) as 1.38 Rs (from the Sun center). The CME height at the time of GLE particle release was directly measured from a STEREO image as 2.32 Rs, which agrees well with the estimation from CME kinematics. These heights are consistent with those obtained for cycle-23 GLEs using back-extrapolation. By contrasting the 2012 May 17 GLE with six other non-GLE eruptions from well-connected regions with similar or larger flare size and CME speed, we find that the latitudinal distance from the ecliptic is rather large for the non-GLE events due to a combination of non-radial CME motion and unfavorable solar B0 angle, making the connectivity to Earth poorer. We also find that the coronal environment may play a role in deciding the shock strength.

Authors: N. Gopalswamy, H. Xie, S. Akiyama, S. Yashiro, I. G. Usoskin, and J. M. Davila
Projects: GOES X-rays ,Neutron Monitors,SDO-AIA,SoHO-LASCO,STEREO

Publication Status: Accepted for publication, ApJL, February 6, 2013
Last Modified: 2013-02-07 00:09
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A Tell-Tale Sign of a Wimpy Solar Cycle: the First GLE Event of Solar Cycle 24  

Nat Gopalswamy   Submitted: 2013-01-05 05:31

The current cycle 24 has produced only one GLE event so far on May 17, 2012, whereas cycle 23 had produced five of the 16 GLEs in the frst 4.5 years. The Sun is already in its solar maximum phase, which means it did not produce any GLE event during its rise phase. The lone GLE event is consistent with a weak cycle 24: the sunspot number peaked at 97 compared to 170 in cycle 23, indicating that cycle 24 is 40% weaker.

Authors: N. Gopalswamy
Projects: SoHO-LASCO

Publication Status: INQUIRIES OF HEAVEN, WEDNESDAY, AUGUST 29, 2012 DAY 8, IAU General assembly, 2012
Last Modified: 2013-01-21 01:47
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Observations of CMEs and Models of the Eruptive Corona  

Nat Gopalswamy   Submitted: 2013-01-05 05:27

Current theoretical ideas on the internal structure of CMEs suggest that a flux rope is central to the CME structure, which has considerable observational support both from remote-sensing and in-situ observations. The flux-rope nature is also consistent with the post-eruption arcades with high-temperature plasma and the charge states observed within CMEs arriving at Earth. The model involving magnetic loop expansion to explain CMEs without flux ropes is not viable because it contradicts CME kinematics and flare properties near the Sun. The global picture of CMEs becomes complete if one includes the shock sheath to the CSHKP model.

Authors: Nat Gopalswamy
Projects: ACE,GOES X-rays ,SDO-AIA,SoHO-EIT,SoHO-LASCO,STEREO,Yohkoh-SXT

Publication Status: Solar wind 13, in press, 2013
Last Modified: 2013-01-05 05:50
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Energetic Particle and Other Space Weather Events of Solar Cycle 24  

Nat Gopalswamy   Submitted: 2013-01-05 05:05

We report on the space weather events of solar cycle 24 in comparison with those during a similar epoch in cycle 23. We find major differences in all space weather events: solar energetic particles, geomagnetic storms, and interplanetary shocks. Dearth of ground level enhancement (GLE) events and major geomagnetic storms during cycle 24 clearly standout. The space weather events seem to reflect the less frequent solar eruptions and the overall weakness of solar cycle 24.

Authors: Nat Gopalswamy
Projects: GOES X-rays ,Nobeyama Radioheliograph,SoHO-LASCO

Publication Status: In Space Weather: The space Radiation Environment, Ed. Q. Hu, G. Li, G. P. Zank, X. Ao, O. Verkhoglyadova, J. H. Adama, AIP Conf Proc. 1500, pp. 14-19, 2012
Last Modified: 2013-01-05 05:50
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The Solar Connection of Enhanced Heavy Ion Charge States in the Interplanetary Medium: Implications for the Flux-rope Structure of CMEs  

Nat Gopalswamy   Submitted: 2013-01-05 04:47

We investigated a set of 54 interplanetary coronal mass ejection (ICME) events whose solar sources are very close to the disk center (within ?15 degrees from the central meridian). The ICMEs consisted of 23 magnetic cloud (MC) events and 31 non-MC events. Our analyses suggest that the MC and non-MC ICMEs have more or less the same eruption characteristics at the Sun in terms of soft X-ray flares and CMEs. Both types have significant enhancements in charge states, although the non-MC structures have slightly lower levels of enhancement. The overall duration of charge state enhancement is also considerably smaller than that than that in MCs as derived from solar wind plasma and magnetic signatures. We find very good correlation between the Fe and O charge state measurements and the flare properties such as soft X-ray flare intensity and flare temperature for both MCs and non-MCs. These observations suggest that both MC and non-MC ICMEs are likely to have a flux-rope structure and the unfavorable observational geometry may be responsible for the appearance of non-MC structures at 1 AU. We do not find any evidence for active region expansion resulting in ICMEs lacking a flux rope structure because the mechanism of producing high charge states and the flux rope structure at the Sun is the same for MC and non-MC events.

Authors: N. Gopalswamy, P. Makela, S. Akiyama, H. Xie, S. Yashiro, and A. A. Reinard
Projects: ACE,GOES X-rays ,Nobeyama Radioheliograph,SoHO-EIT,SoHO-MDI,SoHO-LASCO,Yohkoh-SXT

Publication Status: Solar Physics, in press, 2013
Last Modified: 2013-01-07 12:46
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Height of Shock Formation in the Solar Corona Inferred from Observations of Type II Radio Bursts and Coronal Mass Ejections  

Nat Gopalswamy   Submitted: 2013-01-05 04:39

Employing coronagraphic and EUV observations close to the solar surface made by the Solar Terrestrial Relations Observatory (STEREO) mission, we determined the heliocentric distance of coronal mass ejections (CMEs) at the starting time of associated metric type II bursts. We used the wave diameter and leading edge methods and measured the CME heights for a set of 32 metric type II bursts from solar cycle 24. We minimized the projection effects by making the measurements from a view that is roughly orthogonal to the direction of the ejection. We also chose image frames close to the onset times of the type II bursts, so no extrapolation was necessary. We found that the CMEs were located in the heliocentric distance range from 1.20 to 1.93 solar radii (Rs), with mean and median values of 1.43 and 1.38 Rs, respectively. We conclusively find that the shock formation can occur at heights substantially below 1.5 Rs. In a few cases, the CME height at type II onset was close to 2 Rs. In these cases, the starting frequency of the type II bursts was very low, in the range 25 ? 40 MHz, which confirms that the shock can also form at larger heights. The starting frequencies of metric type II bursts have a weak correlation with the measured CME/shock heights and are consistent with the rapid decline of density with height in the inner corona.

Authors: N. Gopalswamy, H. Xie, P. Makela, S. Yashiro, S. Akiyama, W. Uddin., A. K. Srivastava, N. C. Joshi, R. Chandra, P. K. Manoharan, K. Mahalakshmi, V. C. Dwivedi, R. Jain and A. K. Awasthi, N. V. Nitta, M. J. Aschwanden, D. P. Choudhary
Projects: SDO-AIA,STEREO

Publication Status: Accepted for publication in Adv. Space Res., January 4, 2013
Last Modified: 2013-01-07 12:45
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Radio-loud CMEs from the disk center lacking shocks at 1 AU  

Nat Gopalswamy   Submitted: 2012-06-29 15:09

A coronal mass ejection (CME) associated with a type II burst and originating close to the center of the solar disk typically results in a shock at Earth in 2-3 days and hence can be used to predict shock arrival at Earth. However, a significant fraction (about 28%) of such CMEs producing type II bursts were not associated with shocks at Earth. We examined a set of 21 type II bursts observed by the Wind/WAVES experiment at decameter-hectometric (DH) wavelengths that had CME sources very close to the disk center (within a central meridian distance of 30 degrees), but did not have a shock at Earth. We find that the near-Sun speeds of these CMEs average to ~644 km s-1, only slightly higher than the average speed of CMEs associated with radio-quiet shocks. However, the fraction of halo CMEs is only ~30%, compared to 54% for the radio-quiet shocks and 91% for all radio-loud shocks. We conclude that the disk-center radio-loud CMEs with no shocks at 1 AU are generally of lower energy and they drive shocks only close to the Sun and dissipate before arriving at Earth. There is also evidence for other possible processes that lead to the lack of shock at 1 AU: (i) overtaking CME shocks merge and one observes a single shock at Earth, and (ii) deflection by nearby coronal holes can push the shocks away from the Sun-Earth line, such that Earth misses these shocks. The probability of observing a shock at 1 AU increases rapidly above 60% when the CME speed exceeds 1000 km s-1 and when the type II bursts propagate to frequencies below 1 MHz.

Authors: N. Gopalswamy, P. Makela, S. Akiyama, S. Yashiro, H. Xie, R. J. MacDowall, M. L. Kaiser
Projects: ACE,GOES Particles,GOES X-rays ,SoHO-EIT,SoHO-LASCO,Wind

Publication Status: Accepted for publication in JGR Space Physics June 29, 2012
Last Modified: 2012-07-02 12:56
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Properties of Ground Level Enhancement Events and the Associated Solar Eruptions during Solar Cycle 23  

Nat Gopalswamy   Submitted: 2012-05-03 19:32

Solar cycle 23 witnessed the most complete set of observations of coronal mass ejections (CMEs) associated with the Ground Level Enhancement (GLE) events. We present an overview of the observed properties of the GLEs and those of the two associated phenomena, viz., flares and CMEs, both being potential sources of particle acceleration. Although we do not find a striking correlation between the GLE intensity and the parameters of flares and CMEs, the solar eruptions are very intense involving X-class flares and extreme CME speeds (average ~2000 km s-1). An M7.1 flare and a 1200 km s-1 CME are the weakest events in the list of 16 GLE events. Most (80%) of the CMEs are full halos with the three non-halos having widths in the range 167 to 212 degrees. The active regions in which the GLE events originate are generally large: 1290 msh (median 1010 msh) compared to 934 msh (median: 790 msh) for SEP-producing active regions. For accurate estimation of the CME height at the time of metric type II onset and GLE particle release, we estimated the initial acceleration of the CMEs using flare and CME observations. The initial acceleration of GLE-associated CMEs is much larger (by a factor of 2) than that of ordinary CMEs (2.3 km s-12 vs.1 km s-12). We confirmed the initial acceleration for two events for which CME measurements are available in the inner corona. The GLE particle release is delayed with respect to the onset of all electromagnetic signatures of the eruptions: type II bursts, low frequency type III bursts, soft X-ray flares and CMEs. The presence of metric type II radio bursts some 17 min (median: 16 min; range: 3 to 48 min) before the GLE onset indicates shock formation well before the particle release. The release of GLE particles occurs when the CMEs reach an average height of ~3.09 Rs (median: 3.18 Rs; range: 1.71 to 4.01 Rs) for well-connected events (source longitude in the range W20 ? W90). For poorly connected events, the average CME height at GLE particle release is ~66% larger (mean: 5.18 Rs; median: 4.61 Rs; range: 2.75 ? 8.49 Rs). The longitudinal dependence is consistent with shock accelerations because the shocks from poorly connected events need to expand more to cross the field lines connecting to an Earth observer. On the other hand, the CME height at metric type II burst onset has no longitudinal dependence because electromagnetic signals do not require magnetic connectivity to the observer. For several events, the GLE particle release is very close to the time of first appearance of the CME in the coronagraphic field of view, so we independently confirmed the CME height at particle release. The CME height at metric type II burst onset is in the narrow range 1.29 to 1.8 Rs, with mean and median values of 1.53 and 1.47 Rs. The CME heights at metric type II burst onset and GLE particle release correspond to the minimum and maximum in the Alfvén speed profile. The increase in CME speed between these two heights suggests an increase in Alfvénic Mach number from 2 to 3. The CME heights at GLE particle release are in good agreement with those obtained from the velocity dispersion analysis (Reames, 2009a,b) including the source longitude dependence. We also discuss the implications of the delay of GLE particle release with respect to complex type III bursts by ~18 min (median: 16 in; range: 2 to 44 min) for the flare acceleration mechanism. A similar analysis is also performed on the delay of particle release relative to the hard X-ray emission.

Authors: N. Gopalswamy, H. Xie, S. Yashiro, S. Akiyama, P. M?kel? and I. G. Usoskin
Projects: SoHO-EIT,SoHO-LASCO,Yohkoh-SXT

Publication Status: Space Sciences Review, in press, 2012
Last Modified: 2012-05-04 15:01
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Behavior of Solar Cycles 23 and 24 Revealed by Microwave Observations  

Nat Gopalswamy   Submitted: 2012-04-11 13:43

Using magnetic and microwave butterfly diagrams, we compare the behavior of solar polar regions to show that (i) the polar magnetic field and the microwave brightness temperature during the solar minimum substantially diminished during the cycle 23/24 minimum compared to the 22/23 minimum. (ii) The polar microwave brightness temperature (b) seems to be a good proxy for the underlying magnetic field strength (B). The analysis indicates a relationship, B = 0.0067Tb ? 70, where B is in G and Tb in K. (iii) Both the brightness temperature and the magnetic field strength show north-south asymmetry most of the time except for a short period during the maximum phase. (iv) The rush-to-the-pole phenomenon observed in the prominence eruption activity seems to be complete in the northern hemisphere as of March 2012. (v) The decline of the microwave brightness temperature in the north polar region to the quiet-Sun levels and the sustained prominence eruption activity poleward of 60oN suggest that solar maximum conditions have arrived at the northern hemisphere. The southern hemisphere continues to exhibit conditions corresponding to the rise phase of solar cycle 24.

Authors: N. Gpalswamy, S.Yashiro, P. Makela, G. Michalek, K. Shibasaki, and D. Hathaway
Projects: Nobeyama Radioheliograph,SoHO-MDI,SoHO-LASCO

Publication Status: Astrophys. J. Lett. in press
Last Modified: 2012-04-12 09:23
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The Strength and Radial Profile of Coronal Magnetic Field from the Standoff Distance of a CME-driven Shock  

Nat Gopalswamy   Submitted: 2011-06-29 16:55

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: N. Galswamy and S. Yashiro
Projects: SoHO-LASCO,STEREO

Publication Status: To be published online on Wednesday, June 29. It will appear in print in the July 20, 2011 issue (736/1)
Last Modified: 2011-06-30 06:51
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Long-duration Low-frequency Type III Bursts and Solar Energetic Particle Events  

Nat Gopalswamy   Submitted: 2010-10-03 09:16

We analyzed the coronal mass ejections (CMEs), flares, and type II radio bursts associated with a set of three complex, long-duration, low-frequency (<14 MHz) type III bursts from active region 10588 in 2004 April. The durations were measured at 1 and 14 MHz using data from Wind/WAVES and were well above the threshold value (>15 minutes) normally used to define these bursts. One of the three type III bursts was not associated with a type II burst, which also lacked a solar energetic particle (SEP) event at energies >25 MeV. The 1 MHz duration of the type III burst (28 minutes) for this event was near the median value of type III durations found for gradual SEP events and ground level enhancement events. Yet, there was no sign of an SEP event. On the other hand, the other two type III bursts from the same active region had similar duration but were accompanied by WAVES type II bursts; these bursts were also accompanied by SEP events detected by SOHO/ERNE. The CMEs for the three events had similar speeds, and the flares also had similar size and duration. This study suggests that the occurrence of a complex, long-duration, low-frequency type III burst is not a good indicator of an SEP event.

Authors: N. Gopalswamy and P. Makela
Projects: SoHO-LASCO

Publication Status: Published on line in ApJL, 2010
Last Modified: 2010-10-04 18:07
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Interplanetary shocks lacking type II radio bursts  

Nat Gopalswamy   Submitted: 2010-01-18 14:06

We report on the radio-emission characteristics of 222 interplanetary (IP) shocks. A surprisingly large fraction of the IP shocks (~34%) is radio quiet (i.e., the shocks lacked type II radio bursts). The CMEs associated with the RQ shocks are generally slow (average speed ~535 km s-1) and only ~40% of the CMEs were halos. The corresponding numbers for CMEs associated with radio loud (RL) shocks are 1237 km s-1 and 72%, respectively. The RQ shocks are also accompanied by lower peak soft X-ray flux. CMEs associated with RQ (RL) shocks are generally accelerating (decelerating). The kinematics of CMEs associated with the km type II bursts is similar to those of RQ shocks, except that the former are slightly more energetic. Comparison of the shock The RQ shocks seem to be mostly subcritical and quasi-perpendicular. The radio-quietness is predominant in the rise phase and decreases through the maximum and declining phases of solar cycle 23. The solar sources of the shock-driving CMEs follow the sunspot butterfly diagram, consistent with the higher-energy requirement for driving shocks.

Authors: N. Gopalswamy, H. Xie, P. Makela, S. Akiyama, S. Yashiro, M. L. Kaiser, R. A. Howard, J.-L. Bougeret
Projects: SoHO-LASCO

Publication Status: Astrophys. J. in press
Last Modified: 2010-01-19 09:08
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Solar Sources of ''Driverless'' Interplanetary Shocks  

Nat Gopalswamy   Submitted: 2010-01-16 06:42

We identify the solar sources of a large number of interplanetary (IP) shocks that do not have a discernible driver as observed by spacecraft along the Sun-Earth line. At the Sun, these ''driverless'' shocks are associated with fast and wide CMEs. Most of the CMEs were also driving shocks near the Sun, as evidenced by the association of IP type II radio bursts. Thus, all these shocks are driven by CMEs and they are not blast waves. Normally limb CMEs produce driverless shocks at 1 AU. But some disk-center CMEs also result in driverless shocks because of deflection by nearby coronal holes. We estimate the angular deflection to be in the range 20 deg - 60 deg. We also compared the influence of nearby coronal holes on a set of CMEs that resulted in magnetic clouds. The influence is nearly three times larger in the case of driverless shocks, confirming the large deflection required.

Authors: N. Gopalswamy, P. Makela, H. Xie, S. Akiyama and S.Yashiro
Projects: SoHO-LASCO

Publication Status: Solar Wind 12, American Institute of Physics Conference Proceedings, in press, 2009
Last Modified: 2010-01-16 07:28
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The CME link to geomagnetic storms  

Nat Gopalswamy   Submitted: 2010-01-16 06:39

The coronal mass ejection (CME) link to geomagnetic storms stems from the southward component of the interplanetary magnetic field contained in the CME flux ropes and in the sheath between the flux rope and the CME-driven shock. A typical storm-causing CME is characterized by (i) high speed, (ii) large angular width (mostly halos and partial halos), and (iii)solar source location close to the central meridian. For CMEs originating at larger central meridian distances, the storms are mainly caused by the sheath field. Both the magnetic and energy contents of the storm-producing CMEs can be traced to the magnetic structure of active regions and the free energy stored in them.

Authors: N. Gopalswamy
Projects: SoHO-LASCO

Publication Status: In Solar and Stellar Variability: Impact on Earth and Planets, Proceedings IAU Symposium No. 264, 2009, H. Andrei, A. Kosovichev & J.-P. Rozelot, eds. (in press)
Last Modified: 2010-01-16 07:28
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Coronal mass ejections and space weather  

Nat Gopalswamy   Submitted: 2009-07-03 15:26

Solar energetic particles (SEPs) and geomagnetic storms are the two primary space weather consequences of coronal mass ejections (CMEs) and their interplanetary counterparts (ICMEs). I summarize the observed properties of CMEs and ICMEs, paying particular attention to those properties that determine the ability of CMEs in causing space weather. Then I provide observational details of two the central issues: (i) for producing geomagnetic storms, the solar source location and kinematics along with the magnetic field structure and intensity are important, and (ii) for SEPs, the shock-driving ability of CMEs, the Alfvén speed in the ambient medium, and the connectivity to Earth are crucial parameters.

Authors: N. Gopalswamy
Projects: SoHO-EIT,SoHO-MDI,SoHO-LASCO,TRACE

Publication Status: Climate andWeather of the Sun-EarthSystem(CAWSES):SelectedPapers from the2007KyotoSymposium, Edited by T. Tsuda, R. Fujii, K. Shibata, and M. A. Geller, pp. 77?120. TERRAPUB, Tokyo, 2009.
Last Modified: 2009-07-04 10:27
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Erratum to ?Solar sources and geospace consequences of interplanetary magnetic clouds observed during solar cycle 23?Paper 1? [J. Atmos. Sol.-Terr. Phys. 70(2?4) (2008) 245?253]  

Nat Gopalswamy   Submitted: 2009-07-03 15:08

One of the figures (Fig. 4) in ??Solar sources and geospace consequences of interplanetary magnetic clouds observed during solar cycle 23 ? Paper 1?? by Gopalswamy et al.(2008, JASTP, Vol.70, Issues 2?4, February 2008, pp. 245?253) is incorrect because of a software error in the routine that was used to make the plot. The source positions of various magnetic cloud (MC) types are therefore not plotted correctly. Fig. 1 replaces Fig.4 in Paper 1 and is now plotted correctly.

Authors: Gopalswamy, N.; Akiyama, S.; Yashiro, S.; Michalek, G.; Lepping, R. P.
Projects: SoHO-EIT,SoHO-LASCO

Publication Status: Journal of Atmospheric and Solar-Terrestrial Physics, Volume 71, Issue 8-9, p. 1005-1009
Last Modified: 2009-07-04 10:28
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Relation Between Type II Bursts and CMEs Inferred from STEREO Observations  

Nat Gopalswamy   Submitted: 2009-07-03 14:59

The inner coronagraph (COR1) of the Solar Terrestrial Relations Observatory (STEREO) mission has made it possible to observe CMEs in the spatial domain overlapping with that of the metric type II radio bursts. The type II bursts were associated with generally weak flares (mostly B and C class soft X-ray flares), but the CMEs were quite energetic. Using CME data for a set of type II bursts during the declining phase of solar cycle 23, we determine the CME height when the type II bursts start, thus giving an estimate of the heliocentric distance at which CME-driven shocks form. This distance has been determined to be ?1.5Rs (solar radii), which coincides with the distance at which the Alfvén speed profile has a minimum value. We also use type II radio observations from STEREO/WAVES and Wind/WAVES observations to show that CMEs with moderate speed drive either weak shocks or no shock at all when they attain a height where the Alfvén speed peaks (?3Rs ? 4Rs). Thus the shocks seem to be most efficient in accelerating electrons in the heliocentric distance range of 1.5Rs to 4Rs. By combining the radial variation of the CME speed in the inner corona (CME speed increase) and interplanetary medium (speed decrease) we were able to correctly account for the deviations from the universal drift-rate spectrum of type II bursts, thus confirming the close physical connection between type II bursts and CMEs. The average height (?1.5Rs) of STEREO CMEs at the time of type II bursts is smaller than that (2.2Rs) obtained for SOHO (Solar and Heliospheric Observatory) CMEs. We suggest that this may indicate, at least partly, the density reduction in the corona between the maximum and declining phases, so a given plasma level occurs closer to the Sun in the latter phase. In two cases, there was a diffuse shock-like feature ahead of the main body of the CME, indicating a standoff distance of 1Rs ? 2Rs by the time the CME left the LASCO field of view.

Authors: Gopalswamy, N.; Thompson, W. T.; Davila, J. M.; Kaiser, M. L.; Yashiro, S.; M?kel?, P.; Michalek, G.; Bougeret, J.-L.; Howard, R. A.
Projects: SoHO-EIT,SoHO-LASCO,STEREO

Publication Status: Solar Phys. in press, 2009
Last Modified: 2009-07-04 10:28
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Abstracts by Author
Anomalous Expansion of Coronal Mass Ejections during Solar Cycle 24 and its Space Weather Implications
Obscuration of Flare Emission by an Eruptive Prominence
Topical Issue in Solar Physics: Flux-rope Structure of Coronal Mass Ejections: Preface
The First Ground Level Enhancement Event of Solar Cycle 24: Direct Observation of Shock Formation and Particle Release Heights
A Tell-Tale Sign of a Wimpy Solar Cycle: the First GLE Event of Solar Cycle 24
Observations of CMEs and Models of the Eruptive Corona
Energetic Particle and Other Space Weather Events of Solar Cycle 24
The Solar Connection of Enhanced Heavy Ion Charge States in the Interplanetary Medium: Implications for the Flux-rope Structure of CMEs
Height of Shock Formation in the Solar Corona Inferred from Observations of Type II Radio Bursts and Coronal Mass Ejections
Radio-loud CMEs from the disk center lacking shocks at 1 AU
Properties of Ground Level Enhancement Events and the Associated Solar Eruptions during Solar Cycle 23
Behavior of Solar Cycles 23 and 24 Revealed by Microwave Observations
The Strength and Radial Profile of Coronal Magnetic Field from the Standoff Distance of a CME-driven Shock
Long-duration Low-frequency Type III Bursts and Solar Energetic Particle Events
Interplanetary shocks lacking type II radio bursts
Solar Sources of ''Driverless'' Interplanetary Shocks
The CME link to geomagnetic storms
Coronal mass ejections and space weather
Erratum to ?Solar sources and geospace consequences of interplanetary magnetic clouds observed during solar cycle 23?Paper 1? [J. Atmos. Sol.-Terr. Phys. 70(2?4) (2008) 245?253]
Relation Between Type II Bursts and CMEs Inferred from STEREO Observations
Type II Radio Emission and Solar Energetic Particle Events
The SOHO/LASCO CME catalog
Solar connections of geoeffective magnetic structures
Statistical Relationship between Solar Flares and Coronal Mass Ejections
Major Solar Flares without Coronal Mass Ejections
The expansion and radial speeds of coronal mass ejections
Halo Coronal Mass Ejections and Geomagnetic Storms
EUV Wave Reflection from a Coronal Hole
Comment on ''Prediction of the 1-AU arrival times of CMEassociated interplanetary shocks: Evaluation of an empirical interplanetary shock propagation model'' by K.-H. Kim et al.
Large Geomagnetic Storms: Introduction to Special Section
Coronal mass ejections, type II radio bursts, and solar energetic particle events in the SOHO era
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Coronal Mass Ejections and Type II Radio Bursts
Geoeffectiveness of halo coronal mass ejections

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MSU Solar Physics.
Max Millennium Science Mail Archive.
Max Millennium Message of the Day Mail Archive.
Max Millennium Flare Catalog

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