Martina Belz Arndt

Total Solar Eclipse Observations of Hot Prominence Shrouds

Using observations of the corona taken during the total solar eclipses of 2006 March 29 and 2008 August 1 in broadband white light and in narrow bandpass filters centered at Fe X 637.4 nm, Fe XI 789.2 nm, Fe XIII 1074.7 nm, and Fe XIV 530.3 nm, we show that prominences observed off the solar limb are enshrouded in hot plasmas within twisted magnetic structures. These shrouds, which are commonly referred to as cavities in the literature, are clearly distinct from the overlying arch-like structures that form the base of streamers. The existence of these hot shrouds had been predicted by model studies dating back to the early 1970s, with more recent studies implying their association with twisted magnetic flux ropes. The eclipse observations presented here, which cover a temperature range of 0.9 to 2 ×106 K, are the first to resolve the long-standing ambiguity associated with the temperature and magnetic structure of prominence cavities.

How reliable are coronal hole temperatures deduced from observations

Given the importance of the temperature at the base of the corona for the modeling of the solar wind, we investigate the range of temperatures which have been deduced from remote measurements in coronal holes, within a heliocentric distance of 1.6 solar radii, and the accuracy to which these temperatures have been inferred. Results are presented from an analysis of EUV observations made simultaneously in three spectral lines at the limb in a polar coronal hole, with little contamination from quiet region emission. A temperature range of 7.8-9.3 x 10 exp 5 K is obtained, between 1.02 to 1.07 solar radii for the coronal hole, with a very different temperature range of 9.4 x 10 exp 5 - 1.2 x 10 exp 6 K for the quiet regions bordering it. Inhomogeneities within the coronal hole contribute to a 14 percent variation in inferred temperature. The elemental abundance, which is one of the parameters that influence the temperature inference, can in turn be significantly constrained when intensity ratios from three spectral lines are used.

THERMODYNAMICS OF THE SOLAR CORONA AND EVOLUTION OF THE SOLAR MAGNETIC FIELD AS INFERRED FROM THE TOTAL SOLAR ECLIPSE OBSERVATIONS OF 2010 JULY 11

We report on the first multi-wavelength coronal observations, taken simultaneously in white light, Hα 656.3 nm, Feix 435.9 nm, Fex 637.4 nm, Fexi 789.2 nm, Fexiii 1074.7 nm, Fexiv 530.3 nm, and Nixv 670.2 nm, during the total solar eclipse of 2010 July 11 from the atoll of Tatakoto in French Polynesia. The data enabled temperature differentiations as low as 0.2 × 10 6 K. The first-ever images of the corona in Feix and Nixv showed that there was very little plasma below 5 × 10 5 K and above 2.5 × 10 6 K. The suite of multi-wavelength observations also showed that open field lines have an electron temperature near 1×10 6 K, while the hottest, 2×10 6 K, plasma resides in intricate loops forming the bulges of streamers, also known as cavities, as discovered in our previous eclipse observations. The eclipse images also revealed unusual coronal structures, in the form of ripples and streaks, produced by the passage of coronal mass ejections and eruptive prominences prior to totality, which could be identified with distinct temperatures for the first time. These trails were most prominent at 10 6 K. Simultaneous Fex 17.4 nm observations from Proba2/SWAP provided the first opportunity to compare Fex emission at 637.4 nm with its extreme-ultraviolet (EUV) counterpart. This comparison demonstrated the unique diagnostic capabilities of the coronal forbidden lines for exploring the evolution of the coronal magnetic field and the thermodynamics of the coronal plasma, in comparison with their EUV counterparts in the distance range of 1–3 R� . These diagnostics are currently missing from present space-borne and ground-based observatories.

Mapping the Distribution of Electron Temperature and Fe Charge States in the Corona with Total Solar Eclipse Observations

The inference of electron temperature from the ratio of the intensities of emission lines in the solar corona is valid only when the plasma is collisional. Once collisionless, thermodynamic ionization equilibrium no longer holds, and the inference of an electron temperature and its gradient from such measurements is no longer valid. At the heliocentric distance where the transition from a collision-dominated to a collisionless plasma occurs, the charge states of different elements are established, or frozen-in. These are the charge states which are subsequently measured in interplanetary space. We show in this study how the 2006 March 29 and 2008 August 1 eclipse observations of a number of Fe emission lines yield an empirical value for a distance, which we call Rt , where the emission changes from being collisionally to radiatively dominated. Rt ranges from 1.1 to 2.0 R ☉, depending on the charge state and the underlying coronal density structures. Beyond that distance, the intensity of the emission reflects the distribution of the corresponding Fe ion charge states. These observations thus yield the two-dimensional distribution of electron temperature and charge state measurements in the corona for the first time. The presence of the Fe X 637.4 nm and Fe XI 789.2 nm emission in open magnetic field regions below Rt , such as in coronal holes and the boundaries of streamers, and the absence of Fe XIII 1074.7 nm and Fe XIV 530.3 nm emission there indicate that the sources of the solar wind lie in regions where the electron temperature is less than 1.2 × 106 K. Beyond Rt , the extent of the Fe X [Fe9+] and Fe XI emission [Fe10+], in comparison with Fe XIII [Fe12+] and Fe XIV [Fe13+], matches the dominance of the Fe10+ charge states measured by the Solar Wind Ion Composition Spectrometer, SWICS, on Ulysses, at –43° latitude at 4 AU, in March-April 2006, and Fe9+ and Fe10+ charge states measured by SWICS on the Advanced Composition Explorer, ACE, in the ecliptic plane at 1 AU, at the time of both eclipses. The remarkable correspondence between these two measurements establishes the first direct link between the distribution of charge states in the corona and in interplanetary space.

Localized enhancements of Fe+10 density in the corona as observed in Fe Xi 789.2 nm during the 2006 march 29 total solar eclipse

The first ever image of the full solar corona in the Fe XI 789.2 nm spectral line was acquired during the total solar eclipse of 2006 March 29. Several striking features stand out in the processed image: (1) The emission extended out to at least 3 R☉ in streamers. (2) A bubble-like structure, occupying a cone of about 45° and reaching out to 1 R☉ above the limb, was observed southward of a bright active region complex close to the limb. (3) Localized intensity enhancements were found in different parts of the corona at heights ranging from 1.2 to 1.5 R☉. (4) Striations extended out to the edge of the field of view above an almost north-south-oriented prominence. Comparison with the corresponding white-light image taken simultaneously during the eclipse showed no evidence for these localized enhancements, and the bubble-like structure and striations, while present, did not stand out in the same manner. The extent of the Fe XI emission is attributed to the dominance of radiative over collisional excitation in the formation of that spectral line. The localized intensity enhancements, observed only in Fe XI and not in white light, are a signature of localized increases in Fe+10 density relative to electron density. These are the first observations to show direct evidence of localized heavy ion density enhancements in the extended corona. They point to the importance of implementing observations of the Fe XI 789.2 nm line with existing or future coronagraphs for the exploration of the physical processes controlling the behavior of heavy ions in different source regions of the solar wind.

On the Detection of the Signature of Silicon Nanoparticle Dust Grains in Coronal Holes

We report on polarization measurements in the Fe XIII line at 1074.7 nm made during the total solar eclipse of 2001 June 21, which yielded the first signature of interplanetary dust in the inner corona. In the first-ever images at this wavelength, the signature of dust appears as a tangentially polarized emission in the radial extension of the low-temperature and low-density coronal holes as opposed to a predominantly radial polarization direction in the rest of the corona. The observed emission and polarization are attributed to fluorescence from silicon nanoparticle dust grains in the inner corona.

Erratum: “Localized Enhancements of Fe+10 Density in the Corona as Observed in Fe XI 789.2 nm during the 2006 March 29 Total Solar Eclipse” (ApJ, 663, 598 [2007])

The eclipse image of Figure 3 was provided to the authors by Jackob Strikis of the Elizabeth Observatory, Athens, who claimed authorship. However, shortly after publication the authors discovered that this eclipse image was in fact a preliminary version of an image belonging to Prof. Miloslav Druckmuller, taken during the 2006 total solar eclipse from Libya at 30 56.946 0 N, 24 14.3010 E, and at an altitude of 158 m. This image can be found at http://www.zam.fme.vutbr.cz /~druck/Eclipse/index.htm. We extend our gratitude to Prof. Druckmuller, from Brno University of Technology, Czech Republic, who brought this incident to our attention, and who has graciously accepted our apology for this unintentional mishap. A forthcoming article in collaboration with Prof. Druckmuller is in preparation. The Astrophysical Journal, 670:1521, 2007 December 1

The discrete and localized nature of the variable emission from active regions

Using data from the Extreme Ultraviolet (EUV) Spectroheliometer onSkylab, we study the empirical characteristics of the variable emission in active regions. These simultaneous multi-wavelength observations clearly confirm that active regions consist of a complex of loops at different temperatures. The variable emission from this complex has very well-defined properties that can be quantitatively summarized as follows: (1) It is localized predominantly around the footpoints where it occurs at discrete locations. (2) The strongest variability does not necessarily coincide with the most intense emission. (3) The fraction of the area of the footpoints,δn/N, that exhibits variable emission, varies by ±15% as a function of time, at any of the wavelengths measured. It also varies very little from footpoint to footpoint. (4) This fractional variation is temperature dependent with a maximum around 105 K. (5) The ratio of the intensity of the variable to the average background emission, δI/Ī, also changes with temperature. In addition, we find that these distinctive characteristics persist even when flares occur within the active region.

A search for micro cosmic gamma-ray bursts in BATSE one second continuous data

Based on a previous successful search for untriggered solar microflares we have conducted a search for untriggered cosmic gamma-ray bursts (GRBs). The large number of untriggered hard x-ray solar flares suggests that there might exist a class of GRBs with temporal and spectral properties similar to that of hard x-ray solar flares, possibly with similar physics as well. We have scanned a subset of BATSE’s continuous 1 second data from four channels independently. This search can identify GRBs with characteristic rise times longer than 1 second, while the electronic identification system only triggers on rise times of 64-1024 ms. We can also search for softer events, not necessarily SGRs, by including the lowest energy channel. A search of continuous unbiased data can lower the search sensitivity to a minimum determined by the intrinsic and orbit-modulated background. We present the results of our progress.

COMPTEL gamma-ray observations of the C4 solar flare on 20 January 2000

The “Pre-SMM” (Vestrand and Miller 1998) picture of gamma-ray line (GRL) flares was that they are relatively rare events. This picture was quickly put in question with the launch of the Solar Maximum Mission (SMM). Over 100 GRL flares were seen with sizes ranging from very large GOES class events (X12) down to moderately small events (M2). It was argued by some (Bai 1986) that this was still consistent with the idea that GRL events are rare. Others, however, argued the opposite (Vestrand 1988; Cliver, Crosby and Dennis 1994), stating that the lower end of this distribution was just a function of SMM’s sensitivity. They stated that the launch of the Compton Gamma-ray Observatory (CGRO) would in fact continue this distribution to show even smaller GRL flares. In response to a BACODINE cosmic gamma-ray burst alert, COMPtonTELescope on the CGRO recorded gamma rays above 1 MeV from the C4 flare at 0221 UT 20 January 2000. This event, though at the limits of COMPTEL’s sensitivity, clearly shows a nuclear line e...