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Fast single-dish scans of the Sun using ALMA  

Hugh Hudson   Submitted: 2015-04-18 14:19

We have implemented control and data-taking software that makes it possible to scan the beams of individual ALMA antennas to perform quite complex patterns while recording the signals at high rates. We conducted test observations of the Sun in September and December, 2014. The data returned have excellent quality; in particular they allow us to characterize the noise and signal fluctuations present in this kind of observation. The fast-scan experiments included both Lissajous patterns covering rectangular areas, and ?double-circle? patterns of the whole disk of the Sun and smaller repeated maps of specific disk-shaped targets. With the latter we find that we can achieve roughly Nyquist sampling of the Band 6 (230 GHz) beam in 60 s over a region 30000 in diameter. These maps show a peak-to-peak brightness-temperature range of up to 1000 K, while the time-series variability at any given point appears to be of order 0.5% RMS over times of a few minutes. We thus expect to be able to separate the noise contributions due to transparency fluctuations fromvariations in the Sun itself. Such timeseries have many advantages, in spite of the non-interferometric observations. In particular such data should make it possible to observe microflares in active regions and nanoflares in any part of the solar disk and low corona.

Authors: Neil Phillips, Richard Hills, Tim Bastian, Hugh Hudson, Ralph Marson, and Sven Wedemeyer
Projects: None

Publication Status: In press for "Revolution in Astronomy with ALMA - the 3rd Year -
Last Modified: 2015-04-19 18:28
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Momentum Distribution in Solar Flare Processes  

Hugh Hudson   Submitted: 2014-02-05 13:33

We discuss the consequences of momentum conservation in processes related to solar flares and coronal mass ejections (CMEs), in particular describing the relative importance of vertical impulses that could contribute to the excitation of seismic waves (''sunquakes''). The initial impulse associated with the primary flare energy transport in the impulsive phase contains sufficient momentum, as do the impulses associated with the acceleration of the evaporation flow (the chromospheric shock) or the CME itself. We note that the deceleration of the evaporative flow, as coronal closed fields arrest it, will tend to produce an opposite impulse, reducing the energy coupling into the interior. The actual mechanism of the coupling remains unclear at present.

Authors: Hudson, H. S.; Fletcher, L.; Fisher, G. H.; Abbett, W. P.; Russell, A.
Projects: None

Publication Status: Published; Solar Physics, Volume 277, Issue 1, pp.77-88
Last Modified: 2014-02-06 08:43
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Remote sensing of low-energy SEPs via charge exchange  

Hugh Hudson   Submitted: 2014-02-05 13:29

Charge-exchange reactions at high energies provide new channels for the remote sensing of solar high-energy particles, as demonstrated by the recent detection of 1.8-5 MeV hydrogen atoms from a solar flare. Orrall and Zirker had earlier proposed the detection of low-energy protons via charge-exchange atomic reactions in the solar atmosphere, leading in the simplest case to extended red-wing emission in the Lyman α line. We discuss the analogous process for the He II 304 Å line (for α particles) and also assess the feasibility of the analogous process in the solar wind, whereby ambient He and (C, N, O) ions allow low-energy α particles to undergo resonant charge exchange in the ambient corona and thereby produce 304 Å wing emission close to the acceleration region.

Authors: Hudson, H. S.; MacKinnon, A. L.; Badnell, N. R.
Projects: SDO-EVE

Publication Status: Published; SOLAR WIND 13: Proceedings of the Thirteenth International Solar Wind Conference. AIP Conference Proceedings, Volume 1539, pp. 19-21 (2013)
Last Modified: 2014-02-06 08:43
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Cycle 23 Variation in Solar Flare Productivity  

Hugh Hudson   Submitted: 2014-02-05 13:23

The NOAA listings of solar flares in cycles 21 - 24, including the GOES soft X-ray magnitudes, enable a simple determination of the number of flares each flaring active region produces over its lifetime. We have studied this measure of flare productivity over the interval 1975 - 2012. The annual averages of flare productivity remained approximately constant during cycles 21 and 22, at about two reported M- or X-flares per region, but then increased significantly in the declining phase of cycle 23 (the years 2004 - 2005). We have confirmed this by using the independent RHESSI flare catalog to check the NOAA events listings where possible. We note that this measure of solar activity does not correlate with the solar cycle. The anomalous peak in flare productivity immediately preceded the long solar minimum between cycles 23 and 24.

Authors: Hudson, Hugh; Fletcher, Lyndsay; McTiernan, Jim
Projects: RHESSI

Publication Status: Published; Solar Physics, Volume 289, Issue 4, pp.1341-1347
Last Modified: 2014-02-06 08:43
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Charge-exchange Limits on Low-energy α-particle Fluxes in Solar Flares  

Hugh Hudson   Submitted: 2014-02-05 13:18

This paper reports on a search for flare emission via charge-exchange radiation in the wings of the Lyα line of He II at 304 Å , as originally suggested for hydrogen by Orrall & Zirker. Via this mechanism a primary α particle that penetrates into the neutral chromosphere can pick up an atomic electron and emit in the He II bound-bound spectrum before it stops. The Extreme-ultraviolet Variability Experiment on board the Solar Dynamics Observatory gives us our first chance to search for this effect systematically. The Orrall-Zirker mechanism has great importance for flare physics because of the essential roles that particle acceleration plays; this mechanism is one of the few proposed that would allow remote sensing of primary accelerated particles below a few MeV/nucleon. We study 10 events in total, including the γ-ray events SOL2010-06-12 (M2.0) and SOL2011-02-24 (M3.5) (the latter a limb flare), seven X-class flares, and one prominent M-class event that produced solar energetic particles. The absence of charge-exchange line wings may point to a need for more complete theoretical work. Some of the events do have broadband signatures, which could correspond to continua from other origins, but these do not have the spectral signatures expected from the Orrall-Zirker mechanism.

Authors: Hudson, H. S.; Fletcher, L.; MacKinnon, A. L.; Woods, T. N.
Projects: SDO-EVE

Publication Status: Published, ApJ 752, 84 (2012)
Last Modified: 2014-02-06 08:43
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Solar Particle Fluxes and the Ancient Sun  

Hugh Hudson   Submitted: 2011-08-26 08:25

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: Richard E. Lingenfelter and Hugh S. Hudson
Projects: None

Publication Status: In: The ancient Sun: Fossil record in the Earth, Moon and meteorites; Proceedings of the Conference, Boulder, CO, October 16-19, 1979. (A81-48801 24-91) New York and Oxford, Pergamon Press, 1980, p. 69-79.
Last Modified: 2011-08-26 12:58
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Global Properties of Solar Flares  

Hugh Hudson   Submitted: 2011-08-18 07:26

This article broadly reviews our knowledge of solar flares. There is aparticular focus on their global properties, as opposed to the microphysicssuch as that needed for magnetic reconnection or particle acceleration as such.Indeed solar flares will always remain in the domain of remote sensing, so wecannot observe the microscales directly and must understand the basic physicsentirely via the global properties plus theoretical inference. The globalobservables include the general energetics -radiation in flares and mass lossin coronal mass ejections (CMEs) - and the formation of different kinds ofejection and global wave disturbance: the type II radio-burst exciter, theMoreton wave, the EIT 'wave', and the 'sunquake' acoustic waves in the solarinterior. Flare radiation and CME kinetic energy can have comparablemagnitudes, of order 1032 erg each for an X-class event, with the bulk of theradiant energy in the visible-UV continuum. We argue that the impulsive phaseof the flare dominates the energetics of all of these manifestations, and alsopoint out that energy and momentum in this phase largely reside in theelectromagnetic field, not in the observable plasma

Authors: Hugh S. Hudson
Projects: None

Publication Status: Space Science Reviews 158, 5 (2011), DOI 10.1007/s11214-010-9721-4
Last Modified: 2011-08-18 21:56
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The U.S. Eclipse Megamovie in 2017: a white paper on a unique outreach event  

Hugh Hudson   Submitted: 2011-08-18 07:23

Totality during the solar eclipse of 2017 traverses the entire breadth of thecontinental United States, from Oregon to South Carolina. It thus provides theopportunity to assemble a very large number of images, obtained by amateurobservers all along the path, into a continuous record of coronal evolution intime; totality lasts for an hour and a half over the continental U.S. While wedescribe this event here as an opportunity for public education and outreach,such a movie -with very high time resolution and extending to the chromosphere- will also contain unprecedented information about the physics of the solarcorona.

Authors: Hugh S. Hudson, Scott W. McIntosh, Shadia R. Habbal, Jay M. Pasachoff, Laura Peticolas
Projects: None

Publication Status: white paper; companion to arXiv:1108.3486v1
Last Modified: 2011-08-18 21:56
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Generation of electric currents in the chromosphere via neutral-ion drag  

Hugh Hudson   Submitted: 2010-11-29 03:56

We consider the generation of electric currents in the solar chromosphere where the ionization level is typically low. We show that ambient electrons become magnetized even for weak magnetic fields (30 G); that is, their gyrofrequency becomes larger than the collision frequency while ion motions continue to be dominated by ion-neutral collisions. Under such conditions, ions are dragged by neutrals, and the magnetic field acts as if it is frozen-in to the dynamics of the neutral gas. However, magnetized electrons drift under the action of the electric and magnetic fields induced in the reference frame of ions moving with the neutral gas. We find that this relative motion of electrons and ions results in the generation of quite intense electric currents. The dissipation of these currents leads to resistive electron heating and efficient gas ionization. Ionization by electron-neutral impact does not alter the dynamics of the heavy particles; thus, the gas turbulent motions continue even when the plasma becomes fully ionized, and resistive dissipation continues to heat electrons and ions. This heating process is so efficient that it can result in typical temperature increases with altitude as large as 0.1-0.3 eV/km. We conclude that this process can play a major role in the heating of the chromosphere and corona

Authors: V. Krasnoselskikh, G. Vekstein, H.S. Hudson, S.D. Bale, and W.P. Abbett
Projects: None

Publication Status: ApJ 724, 1542 (2010)
Last Modified: 2010-11-29 08:31
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Solar Radiation Belts  

Hugh Hudson   Submitted: 2009-05-21 05:44

The magnetic field of the solar corona has a large-scale dipole character, which maps into the bipolar field in the solar wind. Using standard representations of the coronal field, we show that high-energy ions can be trapped stably in these large-scale closed fields. The drift shells that describe the conservation of the third adiabatic invariant may have complicated geometries. Particles trapped in these zones would resemble the Van Allen belts and could have detectable consequences. We discuss potential sources of trapped particles.

Authors: H. S. Hudson, A. L. MacKinnon, M. L. DeRosa, and S. N. Frewen
Projects: SoHO-MDI

Publication Status: ApJL, to be published 2009
Last Modified: 2009-05-21 09:49
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Flares and the Chromosphere  

Hugh Hudson   Submitted: 2008-08-24 03:56

The chromosphere (the link between the photosphere and the corona) plays a crucial role in flare and CME development. In analogies between flares and magnetic substorms, it is normally identified with the ionosphere, but we argue that the correspondence is not exact. Much of the important physics of this interesting region remains to be explored. We discuss chromospheric flares in the context of recent observations of white-light flares and hard X-rays as observed by TRACE and RHESSI, respectively. We interpret key features of these observations as results of the stepwise changes a flare produces in the photospheric magnetic field.

Authors: H. S. Hudson and L. Fletcher
Projects: RHESSI,TRACE

Publication Status: Earth, Planets, and Space (to be published, 2008)
Last Modified: 2008-08-24 11:20
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Subject will be restored when possible  

Hugh Hudson   Submitted: 2008-02-04 15:31

In this work we study the correlation between the soft (1.6-12.4 keV, mostly thermal) and the hard (20-40 and 60-80 keV, mostly non-thermal) X-ray emission in solar flares up to the most energetic events, spanning about 4 orders of magnitude in peak flux, establishing a general scaling law and extending it to the most intense stellar flaring events observed to date. We used the data from the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) spacecraft, a NASA Small Explorer launched in February 2002. RHESSI has good spectral resolution (~1 keV in the X-ray range) and broad energy coverage (3 keV-20 MeV), which makes it well suited to distinguish the thermal from non-thermal emission in solar flares. Our study is based on the detailed analysis of 45 flares ranging from the GOES C-class, to the strongest X-class events, using the peak photon fluxes in the GOES 1.6-12.4 keV and in two bands selected from RHESSI data, i.e.20-40 keV and 60-80 keV. We find a significant correlation between the soft and hard peak X-ray fluxes spanning the complete sample studied. The resulting scaling law has been extrapolated to the case of the most intense stellar flares observed, comparing it with the stellar observations. Our results show that an extrapolation of the scaling law derived for solar flares to the most active stellar events is compatible with the available observations of intense stellar flares in hard X-rays.

Authors: C. Isola, F. Favata, G. Micela, H. S. Hudson
Projects: None

Publication Status: A&A 472, 261 (2007)
Last Modified: 2008-02-05 09:59
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Subject will be restored when possible  

Hugh Hudson   Submitted: 2008-02-04 15:25

We survey the subject of Coronal Mass Ejections (CMEs), emphasizing knowledge available prior to about 2003, as a synopsis of the phenomenology and its interpretation

Authors: H. S. Hudson, J.-L Bougeret and J. Burkepile
Projects: None

Publication Status: Space Science Reviews 123, 13 ((2006)
Last Modified: 2008-02-05 09:59
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Flare Energy and Magnetic Field Variations  

Hugh Hudson   Submitted: 2007-07-25 12:16

We describe ways in which the photospheric vector magnetic field might vary across the duration of a solar flare or CME. We also quantitatively assess the back reaction on the photosphere and solar interior by the coronal field evolution required to release flare energy. Our estimates suggest that the work done by Lorentz forces in this back reaction could supply enough energy to explain observations of flare-driven seismic waves.

Authors: H.S. Hudson, G.H. Fisher, and B.T. Welsch
Projects: None

Publication Status: NSO Workshop 24, Subsurface and atmospheric influences on solar activity
Last Modified: 2007-07-25 12:17
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The unpredictability of the most energetic solar events  

Hugh Hudson   Submitted: 2007-07-07 19:16

Observations over the past two solar cycles show a highly irregular pattern of occurrence for major solar flares, gamma-ray events, and solar energetic particle (SEP) fluences. Such phenomena do not appear to follow the direct indices of solar magnetic activity, such as the sunspot number. I show that this results from the non-Poisson occurrence for the most energetic events. This Letter also points out a particularly striking example of this irregularity in a comparison between the declining phases of the recent two solar cycles (1993-1995 and 2004-2006, respectively) and traces it through the radiated energies of the flares, the associated SEP fluences, and the sunspot areas. These factors suggest that processes in the solar interior involved with the supply of magnetic flux up to the surface of the Sun have strong correlations in space and time, leading to a complex occurrence pattern that is presently unpredictable on timescales longer than active region lifetimes (weeks) and not correlated well with the solar cycle itself.

Authors: H. S. Hudson
Projects: RHESSI

Publication Status: The Astrophysical Journal, Volume 663, Issue 1, pp. L45-L48.
Last Modified: 2007-07-07 19:49
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Chromospheric Flares  

Hugh Hudson   Submitted: 2007-04-15 17:08

In this topical review I revisit the ''chromospheric flare.'' This should currently be an outdated concept, because modern data seem to rule out the possiblity of a major flare happening independently in the chromosphere alone, but the chromosphere still plays a major observational role in many ways. It is the source of the bulk of a flare's radiant energy - in particular the visible/UV continuum radiation. It also provides tracers that guide us to the coronal source of the energy, even though we do not yet understand the propagation of the energy from its storage in the corona to its release in the chromosphere. The formation of chromospheric radiations during a flare presents several difficult and interesting physical problems.

Authors: H. S. Hudson
Projects: None

Publication Status: Coimbra Solar Physics Meeting on The Physics of Chromospheric Plasmas 2007
Last Modified: 2007-04-16 09:55
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White-light flares: A TRACE/RHESSI overview  

Hugh Hudson   Submitted: 2005-10-19 14:01

The Transition Region and Coronal Explorer (TRACE) instrument includes a ''white light'' imaging capability with novel characteristics. Many flares with such white-light emission have been detected, and this paper provides an introductory overview of these data. These observations have 0.5'' pixel size and use the full broad-band response of the CCD sensor; the images are not compromised by ground-based seeing and have excellent pointing stability as well as high time resolution. The spectral response of the TRACE white-light passband extends into the UV, so these data capture, for the first time in images, the main radiative energy of a flare. This initial survey is based on a sample of flares observed at high time resolution for which the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) had complete data coverage, a total of 11 events up to the end of 2004. We characterize these events in terms of source morphology and contrast against the photosphere. We confirm the strong association of the TRACE white-light emissions - which include UV as well as visual wavelengths - with hard X-ray sources observed by RHESSI. The images show fine structure at the TRACE resolution limit, and often show this fine structure to be extended over large areas rather than just in simple footpoint sources. The white-light emission shows strong intermittency both in space and in time and commonly contains features unresolved at the TRACE resolution. We detect white-light continuum emission in flares as weak as GOES C1.6, limited by photon statistics and background solar fluctuations, and support the conclusion of Neidig (1989) that white-light continuum occurs in essentially all flares.

Authors: H. S. Hudson , C. J. Wolfson, and T. R. Metcalf
Projects: RHESSI,TRACE

Publication Status: Solar Physics in press (2005)
Last Modified: 2005-10-19 14:01
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Coronal loop oscillations and flare shock waves  

Hugh Hudson   Submitted: 2004-08-30 08:54

A statistical analysis of coronal loop oscillations observed by TRACE shows that 12 of 28 cases were associated with metric type II bursts. The timing is consistent with the idea that the loop oscillations in many cases result from the passage of a large-scale wave disturbance originating in a flare in the nearby active region. The GOES classifications for these flares range from C4.2 to X20. Typically the oscillating structures are not disrupted, implying that the disturbance has passed through the medium and it has returned to an equilibrium near that prior to the event. This is consistent with the Uchida interpretation of the disturbance as a weak fast-mode blast wave (i.e., a simple wave at low Alfvénic Mach number) propagating in the ambient corona. We note that all 12 of the associated events were also associated with CMEs, and conclude that the CME eruptions in these cases corresponded to only partial openings of the active-region magnetic fields.

Authors: H. S. Hudson and A. Warmuth
Projects: None

Publication Status: ApJL, to be published 2004
Last Modified: 2004-08-30 08:54
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RHESSI: First Results  

Hugh Hudson   Submitted: 2004-04-26 06:37

The RHESSI observations consist of imaging spectroscopy in the gamma-ray, hard X-ray, and ''firm X-ray'' (3-20 keV) bands. These data are now the most extensive and capable solar high-energy observations at high spectral and spatial resolution. The low-energy hard X-ray spectrum bridges the thermal and non-thermal ranges of solar electron distributions in flares systematically for the first time. In this presentation I survey some results from the first 18 months of observation, including findings on image and spectral morphology (both hard X-ray and gamma-ray, and the behavior of microflares in the 3-20 keV band.

Authors: Hugh S. Hudson
Projects: RHESSI

Publication Status: Solar-B Science Meeting proceedings (2004)
Last Modified: 2004-04-26 06:37
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Total solar irradiance variation during rapid sunspot growth  

Hugh Hudson   Submitted: 2004-04-06 11:26

Large sunspot areas correspond to dips in the total solar irradiance (TSI), a phenomenon associated with the local suppression of convective energy transport in the spot region. This results in a strong correlation between sunspot area and TSI. During the growth phase of a sunspot other physics may affect this correlation; if the physical growth of the sunspot resulted in surface flows affecting the temperature, for example, we might expect to see an anomalous variation in TSI. In this paper we study NOAA active region 8179, in which large sunspots suddenly appeared near disk center, at a time (March 1998) when few competing sunspots or plage regions were present on the visible hemisphere. We find that the area/TSI correlation does not significantly differ from the expected pattern of correlation, a result consistent with a large thermal conductivity in solar convection zone. In addition we have searched for a smaller-scale effect by analyzing white-light images from MDI (the Michelson Doppler Imager) on SOHO. A representative upper-limit energy consistent with the images is on the order of 3 x 1031 ergs, assuming the time scale of the actual spot area growth. This is of the same order of magnitude as the buoyant energy of the spot emergence even if it is shallow. We suggest that detailed image analyses of sunspot growth may therefore show ``transient bright rings'' at a detectable level.

Authors: H. Jabran Zahid, Hugh Hudson, Claus Frohlich
Projects: None

Publication Status: Solar Physics, to be published 2004
Last Modified: 2004-04-06 11:26
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Abstracts by Author
Fast single-dish scans of the Sun using ALMA
Momentum Distribution in Solar Flare Processes
Remote sensing of low-energy SEPs via charge exchange
Cycle 23 Variation in Solar Flare Productivity
Charge-exchange Limits on Low-energy α-particle Fluxes in Solar Flares
Solar Particle Fluxes and the Ancient Sun
Global Properties of Solar Flares
The U.S. Eclipse Megamovie in 2017: a white paper on a unique outreach event
Generation of electric currents in the chromosphere via neutral-ion drag
Solar Radiation Belts
Flares and the Chromosphere
Subject will be restored when possible
Subject will be restored when possible
Flare Energy and Magnetic Field Variations
The unpredictability of the most energetic solar events
Chromospheric Flares
White-light flares: A TRACE/RHESSI overview
Coronal loop oscillations and flare shock waves
RHESSI: First Results
Total solar irradiance variation during rapid sunspot growth
Overview of Solar Flares: The Yohkoh perspective
NOAA 7978: the last best old-cycle region?
X-ray and radio observations of the activation stages of an X-class solar flare
Spectral variations of flare hard X-ray footpoint sources
Coronal scattering as a source of flare-associated polarized hard X-rays
Soft X-ray observation of a large-scale coronal wave and its exciter
Hard X-radiation from a fast coronal ejection
Observing CMEs without coronagraphs
Global coronal waves: implications for HESSI
Hard X-rays from Slow LDEs
Yohkoh and non-thermality in flares
Implosions in Coronal Transients
Coronal Mass Ejections at High Temperatures
The Global Dynamics of the High-Temperature Corona

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