Campaign: Helium Abundance in Flares from the Chromosphere to the Solar Wind.

Authors: G. Poletto, J. Raymond (UVCS); G. Cauzzi, A. Falchi, R. Falciani, L. Smaldone (NSO-Sac Peak);
L. Teriaca, V. Andretta (CDS)

Version:  10 May 2001

Scientific Justification and Objectives:

The behavior of He, the most abundant element after Hydrogen, in different solar structures and/or
events is still not well known.

At chromospheric levels, He abundances in flares have been recently calculated by Mandzhavidze et al.
(1999) using gamma ray spectroscopy: according to these authors, the He/O ratio in Active Regions may
exceed by a factor 2-3 the standard photospheric He abundance.

In the corona, upper limits to the helium abundance have been derived by Raymond et al. (1997), analyzing
UVCS streamer data at an altitude of ~1.5 R_sun. These authors conclude that Helium is less abundant in
streamers than at photospheric levels: hence, they did not find any evidence for a build-up of Helium abundance
in the corona, which is implied by some solar wind models.

Large solar energetic particle (SEP) events and coronal mass ejections (CME) offer a means to infer coronal
abundances, because they drive shock waves that accelerate the coronal plasma up to MeV energies. Analyses
of this kind of events by Ulysses, SOHO and ICI (Ion Composition Instrument, on board the ISEE-3/ICE spacecraft)
experiments led to tentative conclusions: for instance, the correlation between high He abundance and CME,
derived from ecliptic data, may possibly not hold for higher latitude events (Barraclough et al.,1995).
These authors show how Helium enhancements at the ISEE position (in-ecliptic), couldn't be confirmed at
the location of Ulysses (out-of-ecliptic). Also, there may be differences in the He -as well as other
element- abundances depending on whether one looks to the prompt or delayed event component (Reames et al., 1998;
Anttila, 1998; Torsti et al., 2000). Only the depletion of He in the region of sector boundary crossing ( Borrini
et al., 1991; Ogilvie et al., 1992), seems to be a generally accepted, uncontroversial result.

The rather confused scenario that we briefly outlined justifies our proposal for a campaign aimed at
deriving the helium abundance in single events, observed first at chromospheric levels, and later on,
in the corona and solar wind. To this end, we propose to take advantage of the forthcoming Ulysses
quadrature to set up a campaign aimed at catching a solar flare at chromospheric levels and observe
with SOHO and in situ experiments the flare related CME event. This will allow us to determine the
He abundance for the same event from its birth place out to its more remote consequences.
 
 

Campaign dates, Observing schedule and Target selection:

The campaign will be run the week 20-27 May, taking advantage of the SOHO-Ulysses quadrature. At that time,
Ulysses will be at the West limb, at a northern latitude between 2 and 10 degrees. This is also the second
week of MEDOC campaign #7 (http://www.medoc-ias.u-psud.fr )

The prime time for coordinated observations will be during Sac Peak daytime hours:
14-24 UT. Some instruments will offer a more limited coverage (see below).

Needless to say, the cooperation of the Sun, in providing us with a flare/CME in the "right" position,
is essential for the full success of the proposal. The ideal target is obviously an active region likely
to flare near the NW solar limb. However, this might not be an option,  so our list of target selection criteria would
be as follows:
 
1- AR likely to flare within 20 degrees of west limb. A latitude between -30 and +40 will work.
 
2- if the above is not possible, choose an AR likely to flare anywhere on the disk.  Given an equal probability of
flaring, the regions in the NW quadrant should have priority.
 
3- if there is an AR at the NW limb, but its flaring chances are (much) lower than another region on the disk,
choose the latter.

We will rely on the experience of the observers at Big Bear Solar O bservatory  and Kanzelohe Solar Observatory to define the daily target.
A Max Millennium  message of the day (MMmotd) will be sent out daily by the MM Chief Observers
to identify the target region.

A flaring region in a different portion of the solar disk will allow us to obtain at least the chromospheric
part of the study. On the other hand, quadrature observations from SOHO and Ulysses experiment are relevant
in themselves, providing data on the relationship between coronal parameters and the slow wind expected to
originate from the low latitude of the sub-satellite point.
 

Instruments:

NSO-Sac Peak: At chromospheric levels, observations will be performed at the DST of the NSO/Sac Peak Observatory.
sing the DST cluster of instruments, we will obtain the profiles of  HeI lines at 10830 and 5876 A, plus
Halpha and CaII-K to derive the ambient model atmosphere. The FOV is about 4x4 arcmin, and area scans will
be obtained with a cadence of about 30 s. Daily observations will be from 14 to 24 UT, weather permitting.
From the analysis of line profiles, with the support of EUV and SXR data (Yohkoh, TRACE, SOHO-SEM) that
allow a reliable determination of the helium atomic levels kinetics and of the morphology of the region under
examination, the chromospheric He abundance can be derived.

Contact: Gianna Cauzzi (gcauzzi@arcetri.astro.it)
         Roberto Falciani (falciani@arcetri.astro.it)
         Gino Smaldone (smaldone@na.infn.it)
 
 

CDS:  high cadence observations consisting in simultaneous He I 584.3, He II 303.8, Fe XVI 360.8,
Fe XIX 592.2 and O V 629.73 spectroheliograms covering a 148 X 148 arcsec area with a 5.5 minutes
cadence. Each observational run will be opened and closed by spectroheliograms including also density
sensitive line ratios (Fe XIV 334.2 and Fe XIV 353.8).

Contact: Richard Harrison (harrison@solg2.bnsc.rl.ac.uk)
         David Pike (cdp@astro1.bnsc.rl.ac.uk)

SUMER:  SUMER will be switched on during the campaign. There is the possibility that SUMER can look at the
He I line, although not at the time of a flare.
 

UVCS: UVCS has already detected the HeI 584 line. However, if we focus on detecting the He line in a CME,
the situation is challenging, due to the simultaneous presence of several disturbing effects, i.e. the
presence of nearby lines and instrument ghosts, the disappearance of the line at plasma speeds of the order
of 100 km/s, and the position of the line on a vignetted part of the spectrum for several grating position.
However, we feel that the objectives of the proposal are worth a try. Hence, we plan to put the slit,
normal to the radius of the Sun, at a height of about 4 solar radii (west limb, 10 degree latitude,
northern hemisphere) -to minimize vignetting effects- and remain at that altitude throughout the week of
observations. A grtpos of about 236750, with several distinct panels, may be appropriate. Si, OVI, Fe and
carbon lines will be present as well.

Contact: Giannina Poletto (poletto@arcetri.astro.it)
 

SEM: Fluxes in the 0.1-50 nm region are needed in order to estimate the EUV photoionization flux over
the flaring chromosphere. The difference between the flux measured by SEM at the time of a flare and
the daily average will provide the required flux.

Contact: Donald McMullin (mcmullin@usc.edu)
 

LASCO: LASCO will commence its JOP 139 involvement early, beginning 5/01/2001.
C2 pB sequences will be run at a 6 hour (rather than 24 hour) cadence. The cadence will be regular, however
the precise times of day for each sequence are as yet TBD. These details will be provided when available.
The campaign will extend beyond the end of JOP 139, through 7/01/2001 (TBC). Thus the campaign will
include the Ulysses equatorial transit and west limb transits in early May, and the total solar eclipse on
June 21. Additional C3 pB sequences, most likely at a 12 hour regular cadence, are still being considered
and discussed, and a decision will be made shortly.

Contact: G. Lawrence (grl@ajcannon.nascom.gsfc.gov)
 

TRACE Trace will obtain images in 1600 and 171, cadence of about 30 s. WL images every 10 cycles. FOV 6.4x6.4

Contact: Jake Wolfson (wolfson@lmsal.com)
          Harry Warren (warren@cfa.harvard.edu)
 

YOHKOH:  2x2 flare mode, about 40 s cadence

Contact: Nariaki Nitta (sxt_co@isass0.solar.isas.ac.jp)
 

EIT: Synoptic images and 195 CME watch (half/full resolution according to  submode).