The Effect of Hydrostatic Weighting on the Vertical Temperature Structure of the Solar Corona

We investigate the effect of hydrostatic scale heights ${\lambda}(T)$ in coronal loops on the determination of the vertical temperature structure $T(h)$ of the solar corona. Every method that determines an average temperature at a particular line-of-sight from optically thin emission (e.g. in EUV or soft X-ray wavelengths) of a mutli-temperature plasma, is subject to the emission measure-weighted contributions $dEM(T)/dT$ from different temperatures. Because most of the coronal structures (along open or closed field lines) are close to hydrostatic equilibrium, the hydrostatic temperature scale height introduces a height-dependent weighting function that causes a systematic bias in the determination of the temperature structure $T(h)$ as function of altitude $h$. The net effect is that the averaged temperature seems to increase with altitude, $dT(h)/dh > 0$, even if every coronal loop (of a multi-temperature ensemble) is isothermal in itself. We simulate this effect with differential emission measure distributions observed by {\sl SERTS} for an instrument with a broadband temperature filter such as {\sl Yohkoh/SXT} and find that the apparent temperature increase due to hydrostatic weighting is of order \Delta T \approx T_0 \times h/r_{\sun}. We suggest that this effect largely explains the systematic temperature increase in the upper corona reported in recent studies (e.g. by Sturrock et al., Wheatland et al., or Priest et al.), rather than being an intrinsic signature of a coronal heating mechanism.Comment: 7 pages, 3 figures. ApJ Letters, accepted 2000 April 6, in pres

Coronal Magnetic Field Measurement from EUV Images made by the Solar Dynamics Observatory

By measuring the geometrical properties of the coronal mass ejection (CME) flux rope and the leading shock observed on 2010 June 13 by the Solar Dynamics Observatory (SDO) mission's Atmospheric Imaging Assembly (AIA) we determine the Alfv\'en speed and the magnetic field strength in the inner corona at a heliocentric distance of ~ 1.4 Rs. The basic measurements are the shock standoff distance (deltaR) ahead of the CME flux rope, the radius of curvature of the flux rope (Rc), and the shock speed. We first derive the Alfv\'enic Mach number (M) using the relationship, deltaR/Rc = 0.81[(gamma-1) M^2 + 2]/[(gamma+1)(M^2-1)], where gamma is the only parameter that needed to be assumed. For gamma =4/3, the Mach number declined from 3.7 to 1.5 indicating shock weakening within the field of view of the imager. The shock formation coincided with the appearance of a type II radio burst at a frequency of ~300 MHz (harmonic component), providing an independent confirmation of the shock. The shock compression ratio derived from the radio dynamic spectrum was found to be consistent with that derived from the theory of fast mode MHD shocks. From the measured shock speed and the derived Mach number, we found the Alfv\'en speed to increase from ~140 km/s to 460 km/s over the distance range 1.2 to 1.5 Rs. By deriving the upstream plasma density from the emission frequency of the associated type II radio burst, we determined the coronal magnetic field to be in the range 1.3 to 1.5 G. The derived magnetic field values are consistent with other estimates in a similar distance range. This work demonstrates that the EUV imagers, in the presence of radio dynamic spectra, can be used as coronal magnetometers.Comment: 25 pages, 6 figures, 2 table

Solar Energetic Particle Events with Short and Long Onset Times

Gradual solar energetic particle (SEP) events, usually attributed to shock waves driven by coronal mass ejections (CMEs), show a wide variety of temporal behaviors. For example, TO, the >10 MeV proton onset time with respect to the launch of the CME, has a distribution of at least an order of magnitude, even when the source region is not far from the so-called well-connected longitudes. It is important to understand what controls TO, especially in the context of space weather prediction. Here we study two SEP events from the western hemisphere that are different in TO on the basis of >10 MeV proton data from the Geostationary Operations Environmental Satellite, despite similar in the CME speed and longitude of the source regions. We try to find the reasons for different TO, or proton release times, in how the CME-driven shock develops and the Alfv\'en Mach number of the shock wave reaches some threshold, by combining the CME height-time profiles with radio dynamic spectra. We also discuss how CME-CME interactions and active region properties may affect proton release times.Comment: 14 pages, 8 figures, accepted for publication in Ap

3He-Rich Solar Energetic Particles in Helical Jets on the Sun

Particle acceleration in stellar flares is ubiquitous in the Universe, however, our Sun is the only astrophysical object where energetic particles and their source flares can both be observed. The acceleration mechanism in solar flares, tremendously enhancing (up to a factor of ten thousand) rare elements like 3He and ultra-heavy nuclei, has been puzzling for almost 50 years. Here we present some of the most intense 3He- and Fe-rich solar energetic particle events ever reported. The events were accompanied by non-relativistic electron events and type III radio bursts. The corresponding high-resolution, extreme-ultraviolet imaging observations have revealed for the first time a helical structure in the source flare with a jet-like shape. The helical jets originated in relatively small, compact active regions, located at the coronal hole boundary. A mini-filament at the base of the jet appears to trigger these events. The events were observed with the two Solar Terrestrial Relations Observatories STEREO on the backside of the Sun, during the period of increased solar activity in 2014. The helical jets may be a distinct feature of these intense events that is related to the production of high 3He and Fe enrichments.Comment: accepted for publication in The Astrophysical Journa