[Paper Review] Monitoring of Sudden Ionospheric Disturbances (SID) from Kolkata (INDIA)

Sandip K. Chakrabarti, Kinsuk Acharyya+6 more|arXiv (Cornell University)|Jan 14, 2005

Ionosphere and magnetosphere dynamics3 citations

TL;DR

This study presents continuous VLF monitoring of sudden ionospheric disturbances (SIDs) caused by solar flares at Kolkata, India, using a custom loop antenna and audio sampling. It demonstrates excellent agreement between observed SID signatures and NASA GOES X-ray flare data, validating low-cost ground-based detection of solar activity via ionospheric perturbations at 18.2 kHz.

ABSTRACT

We report our first results of monitoring sudden ionospheric disturbances (SID). We present data taken continuously for two weeks during 20th Sept. 2002 and 4th Oct. 2002. We compare the effects of solar flares of the VLF signal with those obtained by GOES Satellite of NASA monitoring in X-rays and found excellent agreement.

Motivation & Objective

• To detect and monitor sudden ionospheric disturbances (SIDs) caused by solar flares using low-cost VLF reception in Kolkata, India.
• To compare ground-based VLF signal variations with space-based X-ray observations from NASA GOES satellite.
• To validate the reliability of ground-based VLF monitoring as a proxy for solar flare activity in regions without dedicated space instrumentation.
• To analyze the temporal behavior of ionospheric disturbances during day/night transitions and solar flare events.
• To assess the feasibility of using VLF signals for real-time monitoring of ionospheric disturbances in the D-layer.

Proposed method

• A Gyrator-II type loop antenna, 1 meter on each side, was used to receive VLF signals at 18.2 kHz from the Indian Navy's VTX3 station in Vijayananarayanam.
• The received signal was amplified and sampled at 3.2 samples per second using a Pentium-IV computer's audio card.
• The antenna was oriented toward the south-southwest to optimize signal reception from the VLF transmitter.
• Data were collected continuously from September 20 to October 4, 2002, excluding maintenance and power outages.
• Signal variations were analyzed for sun-rise and sun-set effects, as well as sudden enhancements due to solar flares.
• GOES satellite X-ray flare data (0.5–8.0 Å) were obtained from NASA archives and compared with VLF observations to validate SID detection.

Experimental results

Research questions

• RQ1 Can ground-based VLF monitoring detect sudden ionospheric disturbances caused by solar flares with sufficient temporal resolution?
• RQ2 How do the timing and duration of VLF signal disturbances correlate with X-ray flare peaks observed by GOES satellite?
• RQ3 To what extent do ionospheric D-layer changes at sunrise and sunset modulate VLF signal amplitude in the Kolkata region?
• RQ4 Can low-cost VLF monitoring systems provide reliable proxy data for solar flare activity in regions without direct solar X-ray monitoring?
• RQ5 What is the characteristic duration of SID events following solar flares, as observed via VLF signal perturbations?

Key findings

• The VLF signal showed a sharp drop at sunrise (around 0h10m UT) due to the D-layer descending to ~40 km altitude, consistent with ionospheric physics.
• Sunset effects were less distinct, with the D-layer taking over 90 minutes to fully disappear, indicating a slower decay process.
• Multiple solar flare events on September 29, 30, and October 3, 2002, produced clear SID signatures in the VLF signal, with rise times of ~15 minutes.
• The duration of each SID event was approximately one hour or longer, matching published reports for similar flare classes.
• There was excellent agreement in peak timing and profile shape between the VLF-based SID detections and GOES satellite X-ray flare data.
• Weak VLF signals were masked by the system’s sampling and processing limitations, indicating a detection threshold below which flares could not be resolved.