Time Profiles of Solar Flare Densities

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Nugget
Number: 176
1st Author: Ryan Milligan
2nd Author: Michael Kennedy
Published: 21 May 2012
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Contents

Introduction

In the modern era, we generally think of solar flares as increases in X-ray luminosity on the Sun due to an increase in the temperature and density of the coronal plasma. The increase in temperature is readily inferred from the presence of high-temperature (>10 MK) emission lines in solar flare spectra. Precise values of the coronal electron density (Ne), on the other hand, have been more difficult to ascertain. Quite often these densities are estimated from broadband continuum measurements which yield values of the flare emission measure (EM=f Ne2V) from instruments such as those of RHESSI and GOES. Deconvolving density values from the emission measure requires a knowledge of the volume of emitting plasma (V, obtained from imaging data) and a possible filling factor (f, often assumed to be unity). An accurate measurement of the electron density is important for understanding both the heating and cooling of flare plasmas, and determining the mechanisms responsible.

A more reliable measurement of electron densities can be made using density-sensitive line ratios, which do not require prior knowledge of the emitting volume or the filling factor, under the assumption that one of the lines is derived from a metastable transition. Previous measurements of flare densities have been made using line-ratios formed at quiescent coronal (1-2 MK) temperatures, and are therefore not a good indicator of the densities associated with flaring plasma observed by RHESSI and GOES. The few measurements that have been made using high-temperature lines focused on a single time during the flare often with an integration time of several minutes. In this Nugget we present time profiles of electron density, determined using line ratios with formation temperatures in excess of 10 MK, during an X-class flare using data from the EUV Variablitiy Experiment (EVE) instrument onboard SDO.

Flare Emission Lines

EVE acquires full disk (Sun-as-a-star) EUV spectra every 10 seconds over the 6.5-37 nm (65-370 A) wavelength range using its MEGS-A (Multiple EUV Grating Spectrographs) component with a near-100% duty cycle. The 9-16 nm portion of this wavelength range contains many emission lines from transitions in high-temperature (>10 MK) Fe ions (XVIII-XXIII) as shown in Figure 1. These high temperature iron lines dominate the EVE spectrum during a flare and, theoretically, many of them are sensitive to changes in electron density. However, because EVE has a relatively modest spectral resolution, many of the observed lines are blended, while others are relatively weak except during the largest events.

Plot of the 8-16 nm portion of the EVE spectra prior to the start of the X6.9 flare that occurred on 9 August 2011 (black curve) and at the SXR peak (red curve). Lines formed from transitions of highly-ionized iron are noted.

Flare Densities

Despite EVE's modest resolution, we have identified three Fe XXI line ratios which are sensitive to densities in the range 1010-1014 cm-3. By fitting each emission line with a Gaussian profile and taking the ratio of the fluxes, the corresponding densities can be calculated using the predictions of CHIANTI. Figure 2 shows the time-profiles of electron density for each of the three ratios during the X6.9 flare on 9 August 2011. Also shown is the GOES lightcurve for the event. From the figure it can be seen that consistant peak densities of around 1012 cm-3 are measured for each ratio. Perhaps more significantly though, is that the evolution of the density can be made for about 15 minutes at 10 second cadence. The time of the maximum density also corresponds to the time of peak emission measure as derived from GOES observations, as one might expect since they are both observing the same high-temperature plasma.

Electron density profiles during the X6.9 flare that occurred on 9 August 2011, derived using three Fe XXI line ratios. The top panel shows the GOES lightcurves for the event in the 0.05-0.4 and 0.1-0.8 nm channels, while the bottom three panels show the electron density time profiles derived from the 14.573/12.875, (14.214+14.228)/12.875, and 12.121/12.875 ratios, respectively (solid black lines). Grey lines denote the upper and lower limits, although no measurements could be reliably made below 1011 cm-3. The vertical dashed line in each panel denotes the time of peak emission measure as derived from the GOES data.

Conclusions

EVE now allows us to routinely measure the electron density of the high-temperature plasma associated with solar flares, which may help us understand various aspects of the flaring process. For example, the high densities often inferred from the SXR emitting corona during flares are thought to be a result of chromospheric evaporation. Density values could therefore be useful for determining the mass rate into (and out of) the overlying loops during heating (and cooling). Flare energetics can often be inferred from X-ray observations as the radiated energy scales with the square of the density. Small changes in the density can therefore lead to significant differences in the derived energetics. The filling factor can also be determined by a combination of imaging and spectroscopy once the density is known. EM can be determined from continuum observations from GOES or RHESSI, while the volume can be estimated from imaging instruments such as RHESSI, GOES/SXI, or Hinode/XRT. High coronal densities may also help explain the occurrence of coronal HXR emission also observed by RHESSI.

Biographical Note

Ryan Milligan is currently a Leverhulme Trust Research Fellow working at Queen's University Belfast. Michael Kennedy is an MSci student also at Queen's University Belfast.

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