We report results from a multiwavelength observing campaign conducted during 2000 March on the flare star AD Leo. Simultaneous data were obtained from several ground-and space-based observatories, including observations of eight sizable flares. We discuss the correlation of line and continuum emission in the optical and ultraviolet wavelength regimes, as well as the flare energy budget, and we find that the emission properties are remarkably similar even for flares of very different evolutionary morphology. This suggests a common heating mechanism and atmospheric structure that are independent of the detailed evolution of individual flares. We also discuss the Neupert effect, chromospheric line broadening, and velocity fields observed in several transition region emission lines. The latter show significant downflows during and shortly after the flare impulsive phase. Our observations are broadly consistent with the solar model of chromospheric evaporation and condensation following impulsive heating by a flux of nonthermal electrons. These data place strong constraints on the next generation of radiative hydrodynamic models of stellar flares.

The Ionospheric Connection Explorer, or ICON, is a new NASA Explorer mission that will explore the boundary between Earth and space to understand the physical connection between our world and our space environment. This connection is made in the ionosphere, which has long been known to exhibit variability associated with the sun and solar wind. However, it has been recognized in the 21st century that equally significant changes in ionospheric conditions are apparently associated with energy and momentum The Ionospheric Connection Explorer (ICON) mission Edited by Doug Rowland and Thomas J. Immel B T.J. Immel

Abstract. We present the results of extended monitoring observations in soft X-rays of the bright eclipsing polar (AM Herculis star) HU Aqr. It was observed between 1990 and 1998 by ROSAT for a total of 230 ksec using the PSPC and the HRI detectors and by EUVE with the Deep Survey Imager and the Spectrometer for a total of 580 ksec. The overall X-ray brightness of HU Aqr varied due to changes of the mass accretion rate by a factor of 40 over that period of time. At all occasions the X-ray light curve was characterized by a marked on-off behavior due to the self-eclipse of the accreting pole. The X-ray light curve showed eclipses by the companion star, the accretion stream and by an accretion curtain raised between the two stars in the binary. Narrow dips prior to the stellar eclipse are caused by the transit of the outer accretion stream. These dips display marked phase shifts, thus indicating a large movement of the threading region, where the stream couples to the magnetic field. These shifts are shown to be related to changes of the mass accretion rate. Correspondingly, the spot longitude varied between 34 • and 50 • . The X-ray light curves display clear evidence for the presence of an accretion curtain, which is raised all along the ballistic accretion stream down to the region where the bulk of matter couples onto magnetic field lines. A lower limit to the mass accretion rate in the curtain is 6 × 10 −12 M /yr, which is of order 10% of the total mass accretion rate. A linear fit to all available eclipse egress times yields an updated orbital ephemeris of the system: BJED(T0) = 2449217.345872(35) + E × 0.086820416195(47) with T0 the time of eclipse of the white dwarf centre of mass (BJED: barycentric Julian ephemeris time). The inclusion of a quadratic term gives a better fit to the data but is not regarded as indication of a period change or asynchronous rotation but by a migration of the accretion spot over the surface of the white dwarf. For one particular data set obtained in a high accretion state, detailed light curve modeling was possible. The egress from eclipse lasted 1.3 s, which constrained the azimuthal extent of the accretion spot to less than 4 • or 450 km. The spot extended vertically by ≤0.015 R wd . A comparison of the width of the stream dip and the extent of the accretion spot shows, that only the inner 60-80% of the stream are dense enough to fire the soft X-ray engine. During the eclipse, HU Aqr was detected at a flux level of 6 × 10 −14 erg cm −1 s −1 . The implied X-ray luminosity is LX = 2.2 × 10 29 erg s −1 , comparable with X-ray emission from single, late-type, active stars.

We have observed the intermediate polar EX Hydrae for 180,000 s with the Extreme Ultraviolet Explorer. The EUV Ñux is strongly modulated at both the binary period (98 minutes) and the white dwarf spin period (67 minutes). The secondary star eclipse is total, with a duration of 41^2 s. The ingress and egress of the secondary eclipse have a duration \3 s (68% conÐdence). The centroid of the secondary eclipse is independent of the white dwarf spin phase to within 6 s. The eclipse attributed to the disk bulge has an energy dependence consistent with photoelectric absorption through a neutral column of about 1.3 ] 1020 cm~2. Folded on the white dwarf spin period, the EUV Ñux is modulated by a factor of 3.7. The shape of the light curve can be accurately matched with a simple geometrical absorption model, although the variation in modulation depth with energy (e.g., EUV vs. X-rays) requires a more sophisticated model. The EUV spectrum reveals many narrow emission lines characteristic of a plasma around 107 K, and possibly features indicating temperatures as low as 106 K. Assuming optically thin emission, we Ðnd a volume emission measure around 3 ] 1054 cm~3 for the 107 K plasma, and an accretion rate around 3 ] 1016 g s~1. By direct geometrical arguments we constrain the EUV emission region to a region extending over less than 4% of the white dwarf surface area located within 0.5 white dwarf radii (in the orbital plane) and 4.5 white dwarf radii (perpendicular to the plane) relative to the center of the white dwarf. Two independent arguments lead to the conclusion that the electron density is [1013 cm~3, probably [1015 cm~3, in the EUV-emitting region.

The Cosmic Hot Interstellar Plasma Spectrometer (CHIPS) was designed to study diffuse emission from hot gas in the local interstellar cavity in the wavelength range 90 -265Å. Between launch in January 2003 and early 2004, the instrument was operated in narrow-slit mode, achieving a peak spectral resolution of about 1.4Å FWHM. Observations were carried out preferentially at high galactic latitudes; weighted by observing time, the mean absolute value of the galactic latitude for all narrow-slit observations combined is about 45 degrees. The total integration time is about 13.2 Msec (74% day, 26% night). In the context of a standard collisional ionization equilibrium plasma model, the CHIPS data set tight constraints on the emission measure at temperatures between 10 5.55 K and 10 6.4 K. At 10 6.0 K, the 95% upper limit on the emission measure is about 0.0004 cm −6 pc for solar abundance plasma with foreground neutral hydrogen column of 2 x 10 18 cm −2 . This constraint, derived primarily from limits on the extreme ultraviolet emission lines of highly ionized iron, is well below the range for the local hot bubble estimated previously from soft X-ray studies. If the pattern of elemental depletion in the hot gas follows that observed in much denser interstellar clouds, the gas phase abundance of iron, relative to other heavy elements that contribute more to the soft X-ray emission, might be much lower than solar. However, to support the emission measures inferred previously from X-ray data would require depletions much higher than the moderate values reported previously for hot gas. Excluding the He II Lyman lines, which are known to be primarily terrestrial in origin, the brightest feature we find in the integrated spectrum is an Fe IX line at 171.1Å. The sky-averaged flux of the feature is about 6 photons cm −2 s −1 ster −1 , a flux that exceeds the 1-sigma shot noise significantly but is comparable to the systematic uncertainty. We find "bright" 171.1Å emission (flux greater than 10 photons cm −2 s −1 ster −1 and S/N > 2) in about 10% of the observing time. However, these "bright" observations overwhelmingly select for day time (96% of 1.3 Msec). Thus, a local rather than interstellar origin for much of the 171.1Å emission seems likely.

Pointed EUVE and ROSAT observations of the AM Her type binary OS Tel (RE 1938-461) are reported, together with complementary contemporaneous optical measurements. The EUVE data reveal a double-peaked orbital light curve, dramatically different from the 'bright-faint' morphology seen during the ROSAT WFC survey discovery observations, indicating that two accretion sites were active. A deep dip is present during one of the EUVE flux maxima and probably arises from occultation of the emission site by the accretion flow rather than via an eclipse by the companion star. This dip, which does not appear to be accompanied by significant spectral hardening, possesses a slow ('" 300 s) ingress but much more rapid ('" 40 s) egress. Both ingress and a restricted phase interval near mid-dip are affected by strong flare-like activity. Blackbody representation of the EUVE spectra yields a low emission temperature", 15 eV. The inferred estimates of the soft component flux point to a large softlhard component flux ratio (~15). However, we also find tentative evidence of an ionization edge at 85 A and absorption lines at 98 and 116 A, possibly due to Ne VI, Ne VIII and Ne VII respectively. Contemporaneous

On 2000 May 5, we began a large multi-wavelength campaign to study the intermediate polar, EX Hydrae. The simultaneous observations from six satellites and four telescopes were centered around a one million second observation with EUVE. Although EX Hydrae has been studied previously with EUVE, our higher signal-to-noise observations present new results and challenge the current IP models. Previously unseen dips in the light curve are reminiscent of the stream dips seen in polar light curves. Also of interest is the temporal extent of the bulge dip; approximately 0.5 in phase, implying that the bulge extends over half of the accretion disk. We propose that the magnetic field in EX Hydrae is strong enough (a few MG) to begin pulling material directly from the outer edge of the disk, thereby forming a large accretion curtain which would produce a very broad bulge dip. This would also result in magnetically controlled accretion streams originating from the outer edge of the disk. We also present a period analysis of the photometric data which shows numerous beat frequencies with strong power and also intermittent and wandering frequencies, an indication that physical conditions within EX Hya changed over the course of the observation. Iron spectral line ratios give a temperature of log T = 6.5 − 6.9 K for all spin phases and a poorly constrained density of n e = 10 10 − 10 11 cm −3 for the emitting plasma. This paper is the first in a series detailing our results from this multi-wavelength observational campaign.

We have developed a model of the high-energy accretion region for magnetic cataclysmic variables and applied it to Extreme Ultraviolet Explorer observations of 10 AM Herculis type systems. The major features of the EUV light curves are well described by the model. The light curves exhibit a large variety of features such as eclipses of the accretion region by the secondary star and the accretion stream, and dips caused by material very close to the accretion region. While all the observed features of the light curves are highly dependent on viewing geometry, none of the light curves are consistent with a flat, circular accretion spot whose lightcurve would vary solely from projection effects. The accretion region immediately above the WD surface is a source of EUV radiation caused by either a vertical extent to the accretion spot, or Compton scattering off electrons in the accretion column, or, very likely, both. Our model yields spot sizes averaging 0.06 R W D , or f ∼ 1 × 10 −3 the WD surface area, and average spot heights of 0.023 R W D . Spectra extracted during broad dip phases are softer than spectra during the out-of-dip phases. This spectral ratio measurement leads to the conclusion that Compton scattering, some absorption by a warm absorber, geometric effects, an asymmetric temperature structure in the accretion region and an asymmetric density structure of the accretion column are all important components needed to fully explain the data. Spectra extracted at phases where the accretion spot is hidden behind the limb of the WD, but with the accretion column immediately above the spot still visible, show no evidence of emission features characteristic of a hot plasma.