A. Geranios

MHD simulation of an interaction of a shock wave with a magnetic cloud

Interplanetary shock waves, propagating in the heliosphere faster than earlier-emitted coronal ejecta, penetrate them and modify their parameters during this interaction. Using two and one half dimensional MHD simulations, we show how a magnetic cloud (flux rope) propagating with a speed 3 times higher than the ambient solar wind is affected by an even faster traveling shock wave overtaking the cloud. The magnetic field increases inside the cloud during the interaction as it is compressed in the radial direction and becomes very oblate. The cloud is also accelerated and moves faster, as a whole, while both shocks (driven by the cloud and the faster interplanetary shock) merge upstream of the cloud. This interaction may be a rather common phenomenon due to the frequency of coronal mass ejections and occurrence of shock waves during periods of high solar activity.

Spheroidal models of magnetic clouds and their comparison with spacecraft measurements

We present here magnetic force-free solutions for spherical, oblate, and prolate clouds and show their magnetic field configurations. It is shown that spheroidal models can fit observed clouds as well as the cylindrical model. The spherical model is free of the limitation of the cylindrical model that allows only reduced increase of the magnetic field to 2x of the boundary value following from properties of the Bessel functions. For the tested cases, the cloud diameters following from the fit are generally larger for the spherical model than for the cylindrical one. An analysis of 14 cases shows that the fit using the spherical model is of a comparable accuracy in comparison with the cylindrical model. Generally, no exact determination of the cloud boundaries has been given up to now. We try to estimate cloud boundaries from the plasma data as an independent check, and compare them with cloud boundaries following from models of magnetic clouds. The boundaries given by the spheroidal models are near irregular temperature increases, and we suggest taking these increases as a possible indicator of the cloud physical boundaries.

Spherical and cylindrical models of magnetized plasma clouds and their comparison with spacecraft data

Abstract Cylindrical and spherical models of force-free magnetic field configurations (known as magnetic clouds) are analysed and compared with events observed by Helios I and Prognoz 10. It is shown that more complicated spherical solutions are able to fit observed data and describe the topology of magnetic clouds.

Evidence for a spheroidal structure of magnetic clouds

We have analysed 14 cases of magnetic clouds identified by R. M. Lepping et al. [1990], who have fitted them with the cylindrical model. We treated cloud magnetic field profiles and compared them with the spheroidal models. We argue that all cases exhibit features of spheroidal clouds, namely, the complete sinusoidal profile of magnetic field components inside the cloud, double-peak and/or plateau-type magnetic field magnitude profiles.

ENERGETIC PARTICLE OBSERVATIONS AND PROPAGATION IN THE THREE-DIMENSIONAL HELIOSPHERE DURING THE 2006 DECEMBER EVENTS

We report observations of solar energetic particles obtained by the HI-SCALE and COSPIN/LET instruments onboard Ulysses during the period of isolated but intense solar activity in 2006 December, in the declining phase of the solar activity cycle. We present measurements of particle intensities and also discuss observations of particle anisotropies and composition in selected energy ranges. Active Region 10930 produced a series of major solar flares with the strongest one (X9.0) recorded on December 5 after it rotated into view on the solar east limb. Located over the South Pole of the Sun, at >72°S heliographic latitude and 2.8 AU radial distance, Ulysses provided unique measurements for assessing the nature of particle propagation to high latitudes under near-minimum solar activity conditions, in a relatively undisturbed heliosphere. The observations seem to exclude the possibility that magnetic field lines originating at low latitudes reached Ulysses, suggesting either that the energetic particles observed as large solar energetic particle (SEP) events over the South Pole of the Sun in 2006 December were released when propagating coronal waves reached high-latitude field lines connected to Ulysses, or underwent perpendicular diffusion. We also discuss comparisons with energetic particle data acquired by the STEREO and Advanced Composition Explorer in the ecliptic plane near 1 AU during this period.

An Experimental Investigation of the Indoor Air Quality in Fifteen School Buildings in Athens, Greece

Abstract The present paper aims at investigating the indoor air quality in fifteen school buildings located in the greater Athens area. Experimental investigations were performed in fifteen different school classrooms and the concentration levels of various pollutants such as CO, CO, TVOC, HCHO, and radon, were measured. Moreover, the experimental investigation included measurements of several environmental parameters such as temperature, relative humidity and air velocity inside each classroom, while ventilation was examined by estimating the air changes using the tracer gas technique. From the above investigation, it was found that the indoor air quality inside the classrooms is strongly related to the number of occupants and their activities. The toxic pollutants such as CO, HCHO, TVOC and radon, measured inside the classrooms, present concentrations that are lower than their relative limit values and hence, they do not create any problem for the occupants. However, frequently, the CO concentration levels were found to be higher than the recommended limits. This can be attributed to insufficient natural ventilation as well as to the absence of mechanical ventilation.

Magnetic traps in the interplanetary medium associated with magnetic clouds

MHD simulations of the propagation of magnetic clouds in the interplanetary medium show that interplanetary magnetic field (IMF) lines, draping around the cloud, are often bent in a complicated way. The magnetic field along these field lines (even on nonbent sections) is not smoothly decreasing with increasing distance from the Sun but usually exhibits several extreme values (minima and maxima). Depressions in the IMF strength may trap energetic particles with suitable energies and pitch angles. These particles may remain trapped (in the expanding region) until the IMF configuration changes. Possible locations of magnetic traps are shown in this paper.

Magnetically closed regions in the solar wind

Interplanetary plasma and magnetic field data collected by Helios-1, Helios-2 and IMP-8 satellites over the periods December 1974–December 1976, January 1976–December 1976 and December 1974–December 1976, respectively, are analysed. From this analysis, we identified 85 about cases in which the proton temperature was very low. In 50 of these cases, the interplanetary magnetic field showed characteristic variations favorable for closed structures in the solar wind.By using the calculated radial temperature gradients as a function of the solar wind speed and the heliocentric distance we were able to identify ‘cold’ protons in the neighborhood of the Sun (0.3 AU).The estimation of the distance at which regions of cold protons are formed (10R⊙) shows that this distance is the same whether we are using solar wind plasma data measured in fixed or in varied heliocentric distances.

Statistical analysis of magnetically closed structures

Abstract An attempt is made for a better identification of magnetically closed structures in the solar wind, using the low proton temperatures between 0.3 and 1.0 astronomical unit (a.u.). The analysis is based on Helios -1 hourly data, which cover the period from its launch (December 1974) up to the end of 1980. All hourly data are examined in order to detect low temperature regions embeded within closed magnetic field lines. The solar wind proton parameters, as the temperatures, densities, velocities, components of the Interplanetary Magnetic Field (IMF) and ratios of magnetic to plasma pressures, were checked, being indicative of such configurations. The temperature variation ( T ) with radial distance ( r ) of the “hot” component in the solar plasma ( v > 550 km s −1 ) does not show an adiabatic variation ( T ∼ r − 4 3 ), but a variation with an exponent close to 0.75 ( T ∼ r − 3 4 ). On the contrary, the “cold” component ( v −1 ) shows a variation close to the adiabatic. In almost all events found, the two main criteria for closed structures (which are reduced temperatures and closed magnetic fields) are fulfilled.

November 17–18, 1975, event: A clue to an internal structure of magnetic clouds?

During November 17–18, 1975, a magnetic cloud was observed by the IMP 8 satellite. The cloud was analyzed in several papers. It draws attention because it is the most clear example where the magnetic field components behave differently from the current single flux rope model. Various models and fits have been presented to explain the magnetic field measurements for this particular event: single-polarity cylindrical flux rope, spheromak, toroidal flux rope, and two subsequent flux ropes (flux rope twins). We critically examine these models and fits and stress that not only magnetic field data but also plasma data must be taken into account. There is a remarkably sharp drop in the density inside the magnetic cloud. The most consistent explanation of the behavior of magnetic field and plasma data for this event is that the magnetic cloud consists of a dual-polarity flux rope with a low density and strong magnetic field core surrounded by an annular region of the same chirality but opposite polarity. An implication of this possibility to explain other magnetic cloud observations is discussed.