EUV Enhancements Research Articles

Ionospheric response to X-class solar flares in the ascending half of the subdued solar cycle 24

The signature of 11 X-class solar flares that occurred during the ascending half of the present subdued solar cycle 24 from 2009 to 2013 on the ionosphere over the low- and mid-latitude station, Dibrugarh (27.5∘N, 95∘E; magnetic latitude 17.6∘N), are examined. Total electron content (TEC) data derived from Global Positioning System satellite transmissions are used to study the effect of the flares on the ionosphere. A nonlinear significant correlation (R 2 = 0.86) has been observed between EUV enhancement (ΔEUV) and corresponding enhancement in TEC (ΔTEC). This nonlinearity is triggered by a rapid increase in ΔTEC beyond the threshold value ∼1.5 (×1010 ph cm−2 s−1) in ΔEUV. It is also found that this nonlinear relationship between TEC and EUV flux is driven by a similar nonlinear relationship between flare induced enhancement in X-ray and EUV fluxes. The local time of occurrence of the flares determines the magnitude of enhancement in TEC for flares originating from nearly similar longitudes on the solar disc, and hence proximity to the central meridian alone may not play the dominating role. Further, the X-ray peak flux, when corrected for the earth zenith angle effect, did not improve the correlation between ΔX-ray and ΔTEC.

Analysis of solar EUV and X-ray flux enhancements during intense solar flare events and the concomitant response of equatorial and low latitude upper atmosphere

We investigate the X ray and UV flux changes during flare events and the corresponding ionospheric response. The study reveals that UV flux enhancement depends on both flare intensity and position on solar disk while X-ray flux enhancement depends only on the former. The study brings out a new result that the E region response to flares directly relates to the X-ray flux enhancement and that it does not exhibit limb effect. Further, the F region response shows limb effect indicating the UV flux control on the same and it also shows seasonal variability due to solar zenith angle variability.

An analysis of thermospheric density response to solar flares during 2001–2006

Previous studies show there are significant thermospheric responses to the two great solar flares on October 28, 2003 (X17.2) and November 4, 2003 (X28). In the present study, we further explored the thermospheric response to all X‐class solar flares during 2001–2006. The observed results show that X5 and stronger solar flares can induce an average enhancement of 10–13% in thermospheric density in latitude 50°S‐50°N within ∼4 h after the flare onset. Many important lines and continua in solar EUV region are optically thick, thus EUV enhancements are smaller for flares located near the solar limb due to absorption by the solar atmosphere. Limb flares induce smaller thermospheric responses, due to the limb effect of solar EUV. The thermospheric density enhancement is much more correlated with integrated EUV flux than with peak EUV flux, with a high correlation coefficient of 0.91, which suggests that thermospheric response is strongly dependent on the total integrated energy into the thermosphere.

3D Kinematic Evolution of Erupting Filaments

We reconstruct the three-dimensional geometry of erupting filaments using observations from different viewpoints. True kinematic evolution of different locations along the main bodies of filaments is investigated. For the erupting filament on 2009 September 26, the highest point is the fastest in the first half hour, and then the points at the low-latitude leg of the filament become the fastest. However, for the event on 2010 August 1, the highest location always has the largest velocity and acceleration during the eruption process. The propagation of EUV enhancements at the endpoints of the erupting filament is observed clearly, which is attributed to the successive down-flows along the different groups of thread-like structures of the filament. Both the two filaments show non-radial eruptions and move towards low-latitude region. Their latitudinal variation is greater than the longitudinal.

Statistical analysis of solar EUV and X-ray flux enhancements induced by solar flares and its implication to upper atmosphere

The 0.1-0.8 nm X-ray flux data and 26-34 nm EUV flux data are used to statistically analyze the relationship between enhancement in X-ray flux and that in EUV flux during solar flares in 1996-2006. The EUV enhancement does not linearly increase with X-ray flux from C-class to X-class flares. Its uprising amplitude decreases with X-ray flux. The correlation coefficients between enhancements in EUV and X-ray flux for X, M and C-class flares are only 0.66, 0.58 and 0.54, respectively, which suggests that X-ray flux is not a good index for EUV flux during solar flares. Thus, for studying more accurately solar flare effect on the ionosphere/thermosphere system, one needs to use directly EUV flux measurements. One of important reasons for depressing relationship between X-ray and EUV is that the central meridian distance (CMD) of flare location can significantly affect EUV flux variation particularly for X-class flares: the larger value of CMD results in the smaller EUV enhancement. However, there are much smaller CMD effects on EUV enhancement for M and C-class flares. The solar disc images from SOHO/EIT are utilized to estimate the percentage contribution to total EUV enhancement from the flare region and from other region. The results show the larger percentage contribution from other region for the weaker flares, which would reduce the loss of EUV radiation due to limb location of flare and then weaken the CMD effect for weaker flares like M and C-class.

Variability of thermosphere and ionosphere responses to solar flares

We investigated how the rise rate and decay rate of solar flares affect the thermosphere and ionosphere responses to them. Model simulations and data analysis were conducted for two flares of similar magnitude (X6.2 and X5.4) that had the same location on the solar limb, but the X6.2 flare had longer rise and decay times. Simulated total electron content (TEC) enhancements from the X6.2 and X5.4 flares were 6 total electron content units (TECU) and approximately 2 TECU, and the simulated neutral density enhancements were approximately 15% -20% and approximately 5%, respectively, in reasonable agreement with observations. Additional model simulations showed that for idealized flares with the same magnitude and location, the thermosphere and ionosphere responses changed significantly as a function of rise and decay rates. The Neupert Effect, which predicts that a faster flare rise rate leads to a larger EUV enhancement during the impulsive phase, caused a larger maximum ion production enhancement. In addition, model simulations showed that increased E x B plasma transport due to conductivity increases during the flares caused a significant equatorial anomaly feature in the electron density enhancement in the F region but a relatively weaker equatorial anomaly feature in TEC enhancement, owing to dominant contributions by photochemical production and loss processes. The latitude dependence of the thermosphere response correlated well with the solar zenith angle effect, whereas the latitude dependence of the ionosphere response was more complex, owing to plasma transport and the winter anomaly.

Unusual ionospheric effects observed during the intense 28 October 2003 solar flare in the Brazilian sector

Abstract. The 28 October 2003 solar flare (X-ray Class X17.2) was one of the most intense solar flares observed in the recent past. In the present investigation we show the unusual ionospheric effects observed in the Brazilian sector during this solar flare, using both the ionospheric sounding observations obtained at the UNIVAP stations: Palmas (7–10.2° S, 48.2° W, dip lat. 5.5° S) and Sao Jose dos Campos (23.2° S, 45.9° W, dip lat. 17.6° S), Brazil; and ground-based global positioning system (GPS) data obtained at the "Instituto Brasileiro de Geografia e Estatística" (IBGE) stations: Imperatriz (5.5° S, 47.5° W, dip lat. 2.9° S), Brasilia (15.9° S, 47.9° W, dip lat. 11.7° S), Presidente Prudente (22.3° S, 51.4° W, dip lat. 14.9° S), and Porto Alegre (30.1° S, 51.1° W, dip lat. 20.7° S), Brazil; on two consecutive days, viz., 27 (without solar flare) and 28 (with solar flare) October 2003. It should be mentioned that the vertical total electron content (VTEC) from the GPS observations obtained during the solar flare showed an unusual simultaneous increase in the VTEC values at about 11:00 UT at all four stations associated with the solar flare EUV enhancements and lasted for about 3 h. However, no ionograms were obtained at any of the two UNIVAP stations for a period of about 1 h between about 11:00 to 12:00 UT. Before 11:00 UT (from about 10:45 UT) and after 12:00 UT (to about 16:00 UT), the ionograms were only partial, with the low frequency end missing. During this intense solar flare, hard X-rays (1 to 10 A), as observed by the GOES 12 satellite, were ejected by the Sun during a long period (several hours), with peak radiation at about 11:10 UT. These hard X-ray radiations can penetrate further into the ionosphere, causing an increase in ionization in the lower part of ionosphere (D-region). In this way, the lack of ionograms or partial ionograms, which indicates no echoes or partial echoes of the transmitted digital ionosonde signals, are related to intense absorption, resulting in complete or partial fade-out of radio signals at the lower ionospheric heights. The partial and complete radio fade-out observed in the Brazilian sector for more than six hours must have resulted in considerable difficulties to the radio station operating agencies using ionospheric radio transmissions.

Why are there stationary EIT wave fronts

EIT waves are often observed to be propagating EUV enhancements followed by an expanding dimming region after the launch of CMEs. It was widely assumed that they are the coronal counterparts of the chromospheric Moreton waves, though the former are three or more times slower. The existence of a stationary “EIT wave” front in some events, however, posed a big challenge to the wave explanation. Simulations are performed to reproduce the stationary “EIT wave” front, which is exactly located near the footpoint of the magnetic separatrix, consistent with observations. The formation of the stationary front is explained in the framework of our model where “EIT waves” are supposed to be generated by successive opening of the field lines covering the erupting flux rope in CMEs.

Asymmetric mass accretion in the magnetic cataclysmic variable RE 1149 + 28

We present the first detailed extreme photometric observations of a magnetic cataclysmic variable. Our two Extreme Ultraviolet Explorer (EUVE) observations of the AM Her star RE 1149 + 28 were obtained about 1 yr apart and show light-curve variations on orbital to yearly timescales, as well as long-term mean flux level changes of a factor of 2. The photometric data show a persistent ingress EUV enhancement which lasts approximately 0.04 in phase. We attribute this to a region of approximately 10(exp 3) km in extent at the accretion impact site, on or very near the surface of the white dwarf primary. Our observations of RE 1149 are consistent with a relatively low system inclination and provide a best-fit orbital period of 90.14 +/- 0.015 minutes.

Transient coronal extreme ultraviolet emission before and during the impulsive phase of a solar flare

Transient coronal EUV emission, recorded in an emission line of Fe XI at 180.4 A, has been observed prior to and during the initial stages of a solar flare. This emission provides direct evidence of the interactions between the lower corona and the chromosphere during flare initiation. The first EUV enhancement was correlated with activation and eruption of a filament, extended over a distance of about 120,000 km, and had an emission maximum at the site of filament disruption. A moving source of EUV emission also appeared along the trajectory of the erupting filament as it accelerated through the low corona. The mass of this source is estimated to be greater than that of the initial filament or surrounding corona, implying an additional mass ejection from the chromosphere. EUV emission during the impulsive phase of the flare consisted of transient sources along the photospheric magnetic neutral line, with intervening regions of lesser intensity that are interpreted as transient coronal loops below the erupting filament.

EUV emission, filament activation and magnetic fields in a slow-rise flare

The evolution of coronal and chromospheric structures is examined together with magnetograms for the 1B flare of January 19, 1972. Soft X-ray and EUV studies are based on the OSO-7 data. The Hα filtergrams and magnetograms came from the Sacramento Peak Observatory. Theoretical force-free magnetic field configurations are compared with structures seen in the soft X-ray, EUV and Hα images. Until the flare, two prominent spots were connected by a continuous dark filament and their overlying coronal structure underwent an expansion at the sunspot separation rate of 0.1 km s−1. On January 19, the flare occurred as new magnetic fields emerged at ∼ 1019 Mx h−1 beneath the filament, which untwisted and erupted as the flare began. The pre-flare coronal emissions remained unchanged during the flare except for the temporary addition of a localized enhancement that started 5 min after flare onset. EUV lines normally emitted in the upper transition region displayed a sudden enhancement coinciding in time and location with a bright Hα point, which is believed to be near the flare ‘trigger’ or onset point. The EUV flash and the initial Hα brightening, both of which occurred near the center of the activated filament, were followed by a second EUV enhancement at the end of the filament. The complete disruption of the filament was accompanied by a third EUV enhancement and a rapid rise in the soft X-ray emission spatially coincident with the disappearing filament. From the change of magnetic field inferred from Hα filtergrams and from force-free field calculations, the energy available for the flare is estimated at approximately 1031 erg. Apparently, changes in the overlying coronal magnetic field were not required to provide the flare energy. Rather, it is suggested that the flare actually started in the twisted filament where it was compressed by emerging fields. Clearly, the flare started below the corona, and it appears that it derived its energy from the magnetic fields in or near the filament.