1. Two Years of NoRH and RHESSI Observations: What Have We Learned
Two Years of NoRH and RHESSI Observations: What Have We Learned? What Do We Need To Learn?
Alexander Nindos
Section of Astrogeophysics
Physics Department
University of Ioannina
Ioannina GR-45110
Greece

2. Outline Microwave-HXR primer NoRH-RHESSI morphological studies
Radio and HXR imaging of high-energy electrons
Gyrosynchrotron modeling and HXRs
Motions of HXR footpoint sources
HXR coronal sources

3. Microwave-HXR primer
Both HXR and microwave emissions come from energetic electrons.
MW emission is produced by gyrosynchrotron.
HXR emission is produced by bremsstrahlung.
1. Thin target: in proportion to the ambient density
2. Thick target: in regions of high density (chromosphere).

4. (modified from Nindos et al. 2000)
(modified from White et al. 2002)

5. How it works
MW-emitting and HXR-emitting electrons may or may not come from identical sources.
At the simplest level: acceleration of electrons is a process that injects these electrons onto magnetic field lines at a time dependent rate.
There is always a critical pitch angle (loss cone angle), such that electrons w/ smaller pitch angles are not reflected by the field as they approach the footpoints.
Some fraction of the electrons are injected w/ pitch angles lying in the loss cone, and they will precipitate on their first approach to the footpoint.
The remaining fraction is injected w/ pitch angles outside the loss cone, and they remain trapped in the corona unless strong pitch angle scattering takes place.

6. Pre-RHESSI Morphological Studies
Useful info about the flare configuration. Many studies (e.g. Kundu et al. 1984; Alissandrakis et al. 1988; Wang et al. 1995, Nishio et al. 1997, …).
An example: douple-loop configuration inferred from MW-HXR-SXRs.
(modified from Hanaoka 1999)

7. NoRH-RHESSI Observations of a Big Flare
17 & 34 GHz on
TRACE
RHESSI on
TRACE
(Kundu et al. 2004)

8. Evolution later in the flare

9. Radio & HXR imaging of high-energy electrons
RHESSI offers the opportunity to image HXRs produced by electrons at photon energies above 100 keV.
Using NoRH/RHESSI, we can compare the emissions from electrons in the same energy range at two different regimes: HXRs (free-free) & radio (gyrosynchrotron).
Gyrosynchrotron: very efficient and allows us to detect electrons w/ E hundreds of keV even in small flares.
Flares in which HXRs above 100 keV can be imaged don’t happen so often (the steeply falling power-law spectra don’t yield sufficient photons at high energies for image formation).

10. (from White et al. 2003)
Radio & HXR profiles so similar, we can’t argue that radio comes from a long-lived
population of trapped electrons while the HXRs from directly precipitating electrons,
as occurs in other events (e.g. Raulin et al. 1999). The radio & HXR-emitting electrons
should have a common origin.
A single population of electrons w/ a power-law index of above 100 keV
produces the emission at 17 & 34 GHz if we assume that the GHz radio spectral
index doesn’t reflect the true optically thin value.
Both radio & HXR data require extreme densities of electrons to be accelerated in the
energy release: over 1010 cm-3 above 20 keV!

11. Models of Gyrosynchrotron emission
Bfoot=700 G
Btop=250 G
Azimuth=45o
Footpoint separ.=36”
N=6 x 106 cm-3
Delta=3
Emin=10 keV
Emax=500 keV
Similar models have been developed by
Preka-Papadema & Alissandrakis (1992);
Bastian, Benz, & Gary (1998)

12. Modeling the Gyrosynchrotron emission from a simple flare
(from Kundu, Nindos, Grechnev 2004)

13. Before convolution
Models convolved w/ NoRH beams

14. Gyrosynchrotron Modeling and HXRs
Need to model the time profile of the g-s emission (not only at the flare maximum).
In general, one cannot use the HXR profile as an injection function for the radio-emitting electrons.
The equation for the radio flux due to trapped population is an integral over the trapped particle energy distribution convolved w/ the synchrotron emissivity.
The radio emission from directly precipitating electrons will not have the same emissivity as the trapped electrons (the precipitating electrons have a different pitch angle distribution).

15. At least can we do something with the spectrum?
An observed radiation spectrum represents a combination of acceleration & transport effect, which cannot, in general be separated.
In the case where the transport process is dominated by Coulomb collisions we may possibly separate one from the other (the involved physics is well known).
In the optically thin case, the MW spectrum is a measure of the electron E distribution & can be used as a tool for exploring electron evolution.
First select an event dominated by Coulomb collisions (from the observed spectral flattening & MW morphology in comparison w/ magnetic fields (Lee & Gary 2000).
They explained the temporal evolution of the MW spectrum of such an event if Coulomb collisions act on an initial narrow pitch angle distribution.
Many events show evidence of more extreme pitch angle diffusion than can be provided by Coulomb collisions.

16. HXR Source Motions: Are MWs Relevant?
(modified from
Krucker et al 2003)
Footpoint source f1 moves systematically for more than 10 minutes, but others do
not. This is NOT what would be expected from simple reconnection models.
The absence of footpoint motion in one ribbon might have been explained if the
magnetic configuration there, is more complex.
Since g-s emission depends strongly on the magnetic field, comparison of
suitable MW data (preferably simple events) with RHESSI data may shed more
light on such problems.

17. HXR Coronal Sources
The production of HXRs is proportional to the products of the densities of the ambient thermal electrons and the nonthermal electrons  one of these densities must peak at the loop top.
HXRs may be due to thick-target free-free from electron beams colissionally stopped within the loop because of high loop column density.
A HXR source confined to the loop top requires trapping of the nonthermal electrons there (possibly due to a combination of large pitch angles for the nonthermal electrons and a strong gradient in magnetic field from the loop top to the bottom in order for magnetic mirroring to be effective).
Relation of the mechanism producing coronal HXRs wrt the process giving MW flaring loop emission??

18. Conclusion
We have reached a new era in flare physics where the quality of both radio & HXR data should force us to far more sophisticated models than those we have been using.
Nobeyama has been, and always will be, a valuable source of interesting data.
With the development of FASR, imaging spectroscopy in radio will be achieved, resulting to great progress in flare physics.