[1] A case of the drastic effects of an eastward prompt penetration and a westward overshielding electric field successively affecting the daytime equatorial ionosphere during the space weather event that occurred on 24 November 2001 is presented. Under the influence of the strong eastward prompt penetration electric field starting from 11:25 Indian standard time (IST), the equatorial electrojet (EEJ) strength reached the maximum value of 225 nT at 12:42 IST, almost 7 times greater than the monthly quiet time mean at the same time. This peak EEJ value exceeds the maximum observed values during the month of November for the entire solar cycle by more than 100 nT, irrespective of quiet or disturbed conditions. Further, owing to an ensuing overshielding event that occurred during the main phase of the storm rather than the end of the main phase, this unusually large EEJ showed an equally strong polarity reversal along with a weakening of the sporadic E layer over the equator. The EEJ strength was reduced from +225 to À120 nT at $13:45 IST, resulting in a strong counter electrojet condition. The latitudinal variation of the F region electron density data from the CHAMP satellite reveal an ill-developed equatorial ionization anomaly at 17:00 IST (11:24 UT) over the Indian sector due to this significant weakening of the zonal electric field. These observations showcase the significant degree to which the low-latitude ionosphere can be affected by the interplanetary electric field.
Simultaneous measurements of F‐region vertical drift are made in the evening hours (1700–2100 IST) at Trivandrum (dip 0.6°N) and Kodaikanal (dip 4°N) on fifteen days during December 1993–January 1994 using the HF phase path technique on two different probing frequencies. The data are used to study the height dependence of vertical plasma drift in the bottomside F‐region in the dusk sector after correcting the drifts (at Kodaikanal) for meridional wind effects and chemical loss. It is found that growth and decay of a positive height gradient in vertical drift occurs fairly regularly in the dusk period. On the average the vertical velocity gradient is positive in in the interval 1815–1925 IST and is preceded by negative values. The positive height gradient of vertical plasma drift below the F layer peak is interpreted in terms of altitude dependence of the relative contributions of E and F region dynamos to the electric fields responsible for plasma drifts (vertical and zonal) of the dusktime equatorial F‐region. These results are for winter solstice solar minimum conditions.
Abstract. Fractal geometry is becoming increasingly important in the study of image characteristics. For recognition of regions and objects in natural scenes, there is always a need for features that are invariant and they provide a good set of descriptive values for the region. There are many fractal features that can be generated from an image. In this paper, fractal signatures of nearby galaxies are studied with the aim of classifying them. The fractal signature over a range of scales proved to be an efficient feature set with good discriminating power. Classifiers were designed using nearest neighbour method and neural network technique. Using the nearest distance approach, classification rate was found to be 92%. By the neural network method it has been found to increase to 95%.
Fractal concepts are used to describe the irregular structures and regions of interest of solar images. The most common and easiest way to extract regions of interest from an image is through segmentation. Segmentation techniques vary from conventional edge-detection mechanism to fuzzy c-means clustering. In this study, the pixelwise local fractal dimension of solar images is computed by different techniques. This is followed by different segmentation procedures including the fuzzy-based approach, for extracting the active regions from chromospheric images and assessing their performance. These techniques have also been applied on solar images to extract active regions from Solar Heliospheric Observatory (SOHO) Extreme Ultraviolet Telescope (EIT) images.
This paper investigates the response of the equatorial, and near equatorial, ionosphere to geomagnetic disturbances during the period November [8][9][10] 2004. Ionosonde data from Trivandrum (8.5• N 77• E and dip 0.5• N) and SHAR (13.5 • N, 80.2 • E, dip ∼5.5• N), magnetic field data from Tirunelveli (8.7• N, 76.9• E, dip latitude 0.5 • S) and Alibag (18.64 • N, 72.87• E), and GUVI O/N 2 data in the Indian longitude sector, are used for the study. The behavior of interplanetary parameters is also examined in conjunction with the ionospheric data. On 8 November, the EIA around noontime is not fully inhibited even though the electrojet strength an indicates inhibition of EIA due to a disturbance dynamo electric field effect. It is the enhanced O/N 2 over TRV and SHAR, with a larger increase over SHAR, which results in a larger (than expected) value of the EIA proxy parameter. On 9 November, the comparable values of f o F 2 at TRV and SHAR around noon time is due to the combined effect of a weakened anomaly in the presence disturbance dynamo electric field effects leading to the EIA crest being near SHAR, and increased O/N 2 values at TRV and SHAR with a larger increase at TRV. On 10 November, the very strong values of the EIA proxy-SHAR parameter is attributed to the combined effects of prompt penetration electric field related modulations of EIA, and significant O/N 2 changes at the equatorial, and near equatorial, latitude. Thus, the study reveals the important role of storm-induced O/N 2 changes, along with prompt penetration electric fields and disturbance dynamo electric fields in modulating the ionization distribution in the equatorial ionization anomaly (EIA) region during this period.
[1] A multifrequency HF Doppler radar installed at the magnetic equatorial station Trivandrum provides an opportunity to study the height gradient in vertical plasma drift at the bottomside of equatorial F region during evening time. The multifrequency radar gives near-simultaneous observation of vertical plasma drift at three close by F region heights above the sounding station. The height gradient of the vertical drift shows a negative value during the prereversal enhancement (PRE) period and turns to positive value after the prereversal enhancement. The average height gradient in vertical plasma drift remains negative around PRE and its magnitude decreases with altitude, below F peak. This could be a clear-cut manifestation of the curl-free nature of the low-latitude electric field, and it could also indicate a partial signature of the postsunset velocity vortex at the equatorial F region. The magnitude of the mean height gradient around PRE exhibits a seasonal variation.
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