Recent Solar Maximum Mission (SMM) observations have shown fast and simultaneous increases in hard X-rays (HXR, E25 keV) and ultraviolet continuum (UVC, lambda lambda approx. equals 1600 and 1388 A) radiation. A simple and natural explanation is given for this phenomenon to happen, which does not involve extreme conditions for energy transport processes, and confirms earlier results on the effect of XUV photoionization in the solar atmosphere

Machado, Marcos E.

Mauas, Pablo J.

English

NASA Technical Reports Server

N87-21812
FAST TEMPORAL CORRELATION BETWEEN HARD X-RAY AND ULTRAVIOLET
CONTINUUM BRIGHTENINGS
Marcos E. Machado*
Space Science Laboratory, ES52
NASA Marshall Space Flight Center
Alabama 35812, U.S.A.
Pablo J. Mauas
Instituto de Astronomia y Fisica del Espacio
Buenos Aires, Argentina
ABSTRACT
Recent Solar Maximum Mission (SMM) observations have shown fast and
simultaneous increases in hard X-rays (HXR, E>25 keV) and ultraviolet
continuum (UVC, kX _ 1600 and 1388 A) radiation. We give a simple and natural
explanation for this phenomenon to happen, which does not involve extreme
conditions for energy transport processes, and confirms earlier results on the
effect of XUV photoionizatlon in the solar atmosphere.
i. INTRODUCTION
Recent SMM observations, see Orwig and Woodgate elsewhere in these
proceedings, have shown that besides the close temporal correlation between UV
line emission originating in the transition zone (TZ) and HXR emission at E >
30 keV (Woodgate et al., 1983), a similar relationship exists between HXR's
and ultraviolet continuum (UVC). As shown by Orwig and Woodgate, the
correlation holds in some cases to better than 0.I s, with details that can be
found in their paper, thus extending the relationship between HXR and UV
emission to radiation originating in deep atmospheric levels around the
temperature minimum region (TMR).
As shown by Vernazza et al. (1976, 1981) for the quiet Sun and Machado et
al. (1980) and Machado and H@noux (1982) for the case of flare atmospheres,
the continuum radiation within %k 1350 -1680 A originates close to the TMR and
is primarily due to Sil. It is also known (Vernazza et al., 1981) that the
source function of the silicon continua, with edges at 1682 and 1525 A, is
significantly decoupled from the local Planck function, due to the strong
effect of the photoionization term in the ionization balance equation
4_ f (okJk/hv) dk + C
n(SiIl)/n(Sil) = R ' (I)
*NAS/NRC Research Associate, on leave from CNIE, Argentina.
271 _ _ _ _,,¢ _._]_F_
https://ntrs.nasa.gov/search.jsp?R=19870012379 2020-03-20T11:43:36+00:00Z
where C and R are the collisional ionization and recombination rates
respectively, and o_ is the photoionization cross section which we take from
Vernazza et al. (1976). Since the radiation field that Sil "sees is
characterized, even i_ the quiet Sun, by a radiation temperature larger than
the local electron temperature, Te, it results that the source function of the
continuum, S%, is
SI(Sil) > BI(Te) , (2)
where B% is the Planck function, at and around the optical depth unity
(T = i) level in both continua. This, in turn, leads to emergent intensities
characterized by radiation temperatures larger than the local Te at those
depths.
Therefore, any increase in the UVC radiation, which basically reflects an
increase in SI(Sil) at T = i for each of the continua, may either be due to an
increase in the Jl term of equation (I), which would further increase the S l
decoupllng with respect to B.(T ), or by a strong increase in the collisonal
A e
term C, associated with an increase in the local temperature at the TMR. If
by some means, presumably a large increase in the electron density ne, the C
term becomes dominant under flare conditions, then Sl = B(Te) and the
brightness temperature changes, which have been repo_ted to be ATb > 560 K
(see below) for uniform filling over the observed area (Woodgate, private
communication), would represent local changes in the TMR temperature during
the flare. Such an increase would then imply a change in the internal energy
of the plasma,
AE ffi1.5 nH k AT > 460 erg cm-3 , (3)
where we have taken nH = 4 x 1015 cm-3 at the depth of Sil continuum
formation. Considering that radiative los_ rates , primarily due to H-, can be
estimated to be of the order of I0 erg cm-_ s-_ under such conditions (Machado
et al., 1986; Mauas and Machado, 1986), the large AE value from (3) implies
extremely large energy deposition rates at the TMR, unlikely to be attained by
any canonical energy transport mechanism like accelerated particles or XUV
heating, not to mention heat conduction which is totally ineffective at those
depths (Emslle et al., 1981).
Furthermore, it can be easily demonstrated (Mauas and Machado, 1986) that
nonthermal electron beam ionization of hydrogen (Ricchiazzi and Canfield,
1983), which can lead to a local increase in ne without any accompanying large
change in Te, is rather ineffective at these depths, due to the strong
attenuation of the beam above the TMR.
We are thus left with either the possibility that strong and rapid local
heating, as specified in (3), occurs at the TMR during the HXR/UVC bursts, or
that the observed increase is due to a change in the J. term of equation (I).
A
Bearing in mind the unlikeness of the first, for conventinal energy transfer
conditions, we explore in the next section the effect of the second.
272
2. THE EFFECT OF ULTRAVIOLET IRRADIATION
As shown by Machado and H@noux (1982), the photolonlzatlon term on
O
the right hand side of equation (I) can be decomposed into a Jk term&
reflecting the undisturbed quiet Sun conditions, and an additional J_ term
which corresponds to flare irradiation from lines formed in the chromosphere/
corona TZ. The net effect of J_, which is principally related to the increase
in the CIV llne emission at 1549 A, is to alter the Sill/Sil ionization
balance, through photolonlzatlon from the ID level (1682 A continuum) which is
very effectively coupled through collisions, and thus in detailed balance,
with the most populated IS level (1525 A continuum, see Vernazza et al., 1981;
Machado and H_noux, 1982). Therefore, the effect of the largely increased
flare J_ term is to overlonlze Sil through the TMR, as compared with the
results of equation (i) for undisturbed model atmospheres llke those of
Vernazza et al. (1981).
The effect of the increase in n(Sill)/n(Sil) is twofold, it increases the
depth at which T (continuum) = I in the atmosphere, while at the same time it
enhances the already present decoupllng between Sk and Bk(T), increasing the
inequality of equation (2) and leading to brightness temperatures much larger
than the local Te.
Using spatially averaged values of UV irradiation, I(UV), as measured
by Skylab, Machado and _no_x (1982, see references therein) found that for
I(UV) = 4 x 105 erg cm-Z s-i sr-i the variation of Sk leads to a ATh = 280 K,
without any associated increase in the local temperature as compare_ with the
Vernazza et al. (1976) values. We have now performed a similar calculation
using Vernazza et al. (1981) model C atmospheric parameters, with ad-hoc
constant irradiation values I x 106 < I(UV) _ 5 x 106 erg cm -2 s-I sr -I, which
are more appropriate for flare kernels during the impulsive phase as compared
with the averaged flare decay values of Machado and H_noux. The details of
these straightforward statistical equilibrium calculations, in which we have
kept all model C parameters and ionization equilibrium of elements other than
Sil unchanged, will be reported elsewhere in conjunction with a self
consistent, time varying modeling of specific burst observations (see below)•
It suffices to say here that these increased irradiation values lead to
correspondingly larger equilibrium Tb increases, of the order
360 K < AT b < 690 K, (4)
in both the 1525 and 1682 A continua. These values are to be compared to the
AT b = 560 K as observed at = 1600 A in the 20 May, 1984 event, and ATb = 870 K
at 1388 A in the 24 April, 1985 flare (Woodgate, private communication; see
Orwig and Woodgate in these proceedings)•
Furthermore, the characteristic rate of increase of n(Sill) at the
optical depth unity level in the ID continuum (h = 400 km in the irradiated
model C), is given by
dn(Sill)/dt = 7 8 x i0 I0 dF(UV)/dh = 1.32 x i0 I0 cm-3 s-I (5)
273
for I = 106 erg cm-2 s-I sr -I, for F = 2_I and assuming that the ionizing flux
is concentrated in the CIV llne. This gives, for characteristic silicon
densities at these depths (see Vernazza et al., 1981), a characteristic
e-foldlng time of Sill increases of the order of = 1.7 seconds, and
conmensurably shorter values for larger ionizing fluxes.
3. DISCUSSION
In the preceding section we have reported that increases in the UVC
brightness temperature can be effected, as far as steady state statistical
equilibrium calculations show, by the strong photolonlzatlon effect of
transition zone UV i_radlatlo_ under flarelike conditions. Irradiation fluxes
of the order of = I0 erg cm -_ s- seem to be enough to explain the observed
increases in the UVC brightness and, furthermore, they should occur extremely
rapidly.
On the other hand, we would llke to clearly note here that our results
are of very preliminary nature, since they are based on ad-hoc conditions of
constant UV irradiation, as compared with the spiky nature of the bursts. In
a subsequent study we shall model the HXR-TZ emlsslon-UVC correlation for a
set of events, taking into account the observed temporal dependence and
absolute value of the TZ irradiation. In spite of these shortcomings in our
present study, there are a few aspects worth pointing out in the results which
would still stand up after a more detailed study:
a) The strong temporal correlation between TZ and UVC bursts was a
definite prediction of the irradiation model (Machado and H_noux, 1982;
Machado et al., 1986), which has been at least qualitatively confirmed by the
SMM observations.
b) A temporal correlation between HXR and UVC bursts can only be
possible, in our model, through an equally strong correlation between HXR's
and transition zone UV emission. Since this correlation is known to exist
(Tandberg-Hanssen et al., 1983; Woodgate et al., 1983) and to support the
nonthermal thick target model, the HXR/UVC correlation also supports the thick
target model for as much as the previous one does, even though there is no
direct cause-effect relationship between particle heating or ionization and
UVC increases.
c) We further note that the observed increases can simply be due to a
relatively minor energy transport towards the deep atmosphere, as compared
with any model that invokes localized heating at and around the TMR. For
example, instead of the = 107 erg cm -2 s-I needed in our computations,
Aboudarham and H_noux (1986) have found that in order to heat the TMR bv
ele_tro_ beams, the power in the accelerated particles should exceed I0 II erg
cm s for electrons with energies above 20 keV.
274
ACKNOWLEDGEMENTS
We thank A.G. Emslie, E. Tandberg-Hanssen and B.E. Woodgate for useful
discussions and for providing data in advance of publication.
REFERENCES
Aboudarham, J. and Henoux, J.-C., 1986, Astron. Ap., in press.
Emslie, A.G., Brown, J.C. and Machado, M.E., 1981, Ap. J. 246, 337.
Machado, M.E., Emslie, A.G. and Brown, J.C., 1978, Solar Phys. 58, 30.
Machado, M.E. and Henoux, J.-C., 1982, Astron. Ap. 108, 61.
Machado, M.E., Emslie, A.G. and Mauas, P.J., 1986, Astron. Ap., in press.
Mauas, P.J. and Machado, M.E., 1986, Proceedings of the NSO Workshop on the
Physics of the Low Temperature Flare, in press.
Tandberg-Hanssen, E., Reichmann, E. and Woodgate, B.E., 1983, Solar Phys. 86,
159.
Vernazza, J.E., Avrett, E.H. and Loeser, R., 1976, Ap. J. Suppl. 30, i.
, 1981, Ap. J. Suppl. 45, 635.
Woodgate, B.E., Shine, R.A., Poland, A.I. and Orwig, L.E., 1983, Ap. J. 265,
530.
275


Fast temporal correlation between hard X-ray and ultraviolet continuum brightenings

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