Supplementary material to “International Conference on Challenges for Solar Cycle 24”

Debi P. Choudhary, Department of Physics and Astronomy, California State University, Northridge

Citation:
Choudhary, D. P. (2007), International Conference on Challenges for Solar Cycle 24, Eos Trans. AGU, 88(22), 239. [Full Article (pdf)]

Knowledge of the mechanism that governs solar flares and coronal mass ejections is gained from observations in each solar cycle using advanced instrumentation. These observations will be of greatest benefit when planned with prior knowledge gained during the previous activity cycles. With this in mind, the goal of the ICCSC24 was to consider the most effective strategies for studying and understanding the solar energetic events of cycle 24. The conference was attended by about 200 solar and heliospheric researchers from different parts of the world with a dominant Indian contingent. There were a large number of young students and research scholars from Indian Universities.

Solar magnetism and coronal mass ejection, two broad topics, were the central conference themes. Presentations on solar magnetism included active region magnetic field, dynamo mechanisms, flare triggers, and coronal heating mechanisms, while topics related to coronal mass ejection dealt with helioseismological signatures and the associated challenges in understanding large solar energetic particle events. The keynote address by Markus Aschwanden discussed the “Ten highlights and outstanding problems in solar physics”. Among other results he mentioned that at least 10% of the field in the active region is open, rather than looped, through which the particle escape. The heating mechanisms operate at the foot-point of the loops. The solar corona is heated at the base by small scale reconnection processes in the transition region where there is closed magnetic field structure. While, the open field corona is heated by Alfven waves and ion-cyclotron waves that originate at the foot-point of the loops. And, the open field corona is heated by Alfven waves and the ion-cyclotron waves that originate at the foot-point of the loops. The frequency distribution of flare energy obeys the power law over eight orders of magnitude from 1024 to 1032 erg, which is the hallmark for nonlinear processes. The challenge for flare physics is, however, to determine the magnetic field geometry appropriate for reconnection for a given flare event. Still to be understood are the observational signature of nano-flares and pico-flares and the heights at which they occur. The answer to this lies in studying the line profiles as strong signatures for nano-flares can be found in the velocity structure.

The great challenge for solar physics in the coming decade is to understand the source, structure and evolution of solar magnetic fields. Jan Stenflo mentioned, while reviewing solar magnetic fields, that one of the glaring problems of solar magnetism is to understand how the flux is born, where and how it emerges, and how it is removed from the photosphere. What is the fate of all emerged flux? It has to be removed on the solar cycle time scale. One needs to identify the most efficient removal process from

  1. cancellation of opposite polarities (reconnection),
  2. retraction, which is essentially the reprocessing of the flux in the convection zone, and
  3. expulsion through coronal mass ejections.

Another important problem of solar magnetism is understanding why the coronal field rotates almost rigidly while the photospheric field follows a differential rotation.

The most direct way to remotely diagnose or measure the magnetic field on the Sun is through spectro-polarimetry, since magnetic fields imprint polarization signatures in the radiation that is emitted from the solar plasma. Because the physical understanding is found in the small scales, there is a quest to develop large, photon-collecting and highly resolving solar telescopes that allow sensitive spectro-polarimetric imaging at subarcsec scale. Since, however, the magnetic structuring continues on scales that are far smaller than can be resolved in the foreseeable future, new indirect diagnostic techniques based on the combined Hanle and Zeeman effects are being developed to extract statistical properties of the spatially unresolved domain, guided by numerical simulations. Another problem in solar magnetism is to find out to what extent Joy’s law and Hale’s law are valid for ephemeral regions. How does helicity evolve over the solar cycle? Are the polar faculae useful for solar cycle prediction? How does the magnetic field change during a solar flare? Is there an anomalous subsurface structure around N60 degree latitude in the form of a long-lived solar vortex structure? Does the subsurface flow lead to reconnection in the convection zone? An important question for dynamo theory is to explain how much flux is leaked from the Sun. Finally, it is important to understand the relationship between the coronal field and the subsurface magnetic field.

The helioseismological observations have now matured enough to study the subsurface structure of the solar active regions. Frank Hill presented these observations that show the differential rotation of the sun extends down to the tachocline, which has a width of about 0.05 solar radii. There is slow sound speed below the sunspots followed by larger speeds further below. Large shear flows are observed in opposite polarity regions of the sunspots. The rotational energy observed in form of shear flows provides the flare energy. Such studies based on ring diagram analysis can be used to understand the conversing flows for the coronal mass ejection trigger.

Rainer Schwenn stated the definition of CMEs more clearly and listed a number of unanswered questions. A coronal mass ejection is actually the ejection of solar mass as observed in the corona. But how is the ejected plasma cloud integrated into the solar wind? Although recent observations have advanced our knowledge of coronal mass ejections substantially, a number of questions remain unanswered. One of the major issues pertaining to CME observation is the visibility of these events in coronagraphs. Observations with two identical instruments on the STEREO mission could help improve this aspect of CME science.

The list of open questions related to CMEs is large. The most nagging questions in CME science are: what are their pre-event signals? And what is the actual trigger of a CME? CMEs and the flares are symptoms of the same magnetic disease of the Sun. So, what role do flares play for CME’s onset and their further fate? With respect to visibility, how small are the smallest CMEs? Is there a continuous spectrum of CME properties? How long before CME eruption is the big loop formed? How does it hold the material down until it erupts, or is it formed as a part of the eruption process? Where and when are the loop legs ever disconnected? What do the down flow events and collapsing loops really mean? A major question is related to the role of magnetic field reconnection in CMEs: Where and when does reconnection occur and what role does it play: trigger, driver, or sequel? Finally, where, when and how are particles accelerated and how are the CME structures transformed into ICMEs?

Coronal loops are the major observable features considered useful for understanding the nature of large explosive events. X-ray and magnetogram observations show that cool loop filling factors are higher than those of hot loops. In loops the gyro frequency is higher than the collisional frequency, leading to the bumps in distribution function of loop plasma. This makes kinetic theory important for understanding plasma processes compared to MHD theories.

Two competing solar dynamo models were presented by Mausumi Dikpati and Arnab Rai Choudhuri. Dikpati argued for the flux-transport type of solar dynamo, the genesis of which is in the development of mean-field electro-dynamics begun by Parker in 1954. Her model includes meridional circulation that both governs dynamo period and plays a crucial role in determining the Sun’s memory of its past magnetic field. Based on assimilation of magnetic field data from previous cycles, this model predicts a stronger cycle 24 with more sunspots.

The model presented by Choudhuri, on the other hand, uses regular and deterministic processes such as toroidal field generation by differential rotation and advection by meridional circulation. The major difference between the two models is that Dikpati’s model works in the advection-dominated regime, and therefore the memory is governed by the meridional circulation, which provides a 20 year memory of the model, whereas Choudhuri’s model is diffusion-dominated and has very short or no memory. Choudhuri’s model introduces the randomness by the Babcock-Leighton process in the generation of poloidal field and predicts the amount of this field at the end a typical cycle. This theoretical model is corrected by feeding actual observational data of polar field, which however is a fraction of polodial field. For the next solar cycle, it predicts a 35% weaker maximum compared to the present cycle.

Saku Tsuneta presented the data just acquired with the Solar Optical Telescope onboard Hinode and Joe Davila showed the data from the STEREO mission. Planned and working instruments, including a magnetograph for coronal field developed by Haosheng Lin, were also presented at the conference.

Conference proceedings will be published in a special volume of the Journal of Astronomy and Astrophysics which is published by the Indian National Science Academy. Detailed information about the conference can be found at http://www.prl.res.in/~djubconf/.

This summary of the conference is prepared by Debi Prasad Choudhary, Department of Physics and Astronomy, CSUN, Northridge.