JOP139 A STUDY OF HELIUM ABUNDANCE FROM COORDINATED SAC PEAK-SOHO-ULYSSES OBSERVATIONS

Updated: LASCO details added 27 April 2001 by SOC
         CDS, TRACE, Yohkoh parts updated/added 24 May.

Participants: G. Poletto, J. Raymond (UVCS), V. Andretta, L. Teriaca,
K. Mcpherson, R. Harrison, D. Pike, A. Fludra (CDS), J. Newmark,
J. Gurman (EIT), K. Wilhelm (SUMER), G. Lawrence (LASCO), D. McMullin
(SEM), G. Cauzzi, A. Falchi, R. Falciani, L.A. Smaldone (NSO-SAC
PEAK), S. Suess, D.  McComas (Ulysses - SWOOPS), G. Gloeckler,
T. Zurbuchen (Ulysses, SWICS), J.Wolfson, H. Warren (TRACE), N. Nitta
(YOHKOH)

Scientific justification and objectives:

The behaviour 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), analysing 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.

Method and Target selection:
At chromospheric levels, a proposal has been submitted to the NSO/Sac
Peak Observatory (authors: G. Cauzzi (1), A. Falchi (1), R. Falciani
(2), L.A. Smaldone (3), from the Arcetri Astrophysical Observatory (1)
and the University of Firenze (2) and Napoli (3)) meant at using the
DST cluster of instruments to observe the HeI lines at 10830 and 5876
A. 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 He abundance can be derived.  The proposal
indicated the week 20-27 May -plus other time at earlier dates to set
up the experiments- as the time to perform the observations, taking
advantage of the SOHO-Ulysses quadrature. At that time, Ulysses will be
at the West limb, at a northern latitude of about 10 degrees, and the
NSO instruments will be pointing in that area. Their FOV is on the
order of 4 arcmin^2 and observations will be made daily from 14 UT to
24 UT, weather permitting.

Needless to say, the cooperation of the Sun, in providing us with a
flare/CME in the "right" position, is essential for the success of the
proposal. Atthe present phase of the solar cycle, chances for success
are rather high and the area covered by ground based observations is
large enough to guarantee a good probability for flare occurrence. It
should be pointed out, however, that even if the Sun will not be
cooperating, quadrature observations from SOHO and Ulysses experiment
will still be relevant, providing data on the relationship between
coronal parameters and the slow wind expected to originate from the low
latitude of the sub-satellite point.

Operational details:

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 minimise 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.


CDS:
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).

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

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.


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


LASCO:
In addition to routine observations, LASCO may provide extra pB maps at
a 6 hour cadence to help define the morphology of the region under
investigation.  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.

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

YOHKOH:
2x2 flare mode, about 40 s cadence