A Significant Sudden Ionospheric Disturbance Associated with a Massive Gamma-ray Burst

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Nugget
Number:	439
1st Author:	Laura HAYES
2nd Author:	Peter GALLAGHER
Published:	October 31, 2022
Next Nugget:	TBD
Previous Nugget:	Effects of Coronal Structures on the Dynamics of the Global Coronal Wave of SOL2017-09-10
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Contents 1 Introduction 2 Detection of Massive Gamma-ray Burst 221009A in Ionospheric Measurements 3 Conclusions 4 References

Introduction

The Earth's lower ionosphere can act as a giant X-ray and gamma-ray detector from astronomical sources, including the Sun. As a partially ionised plasma, it responds in a characteristic way to ionising radiation from such sources. In particular, the lowest-lying region of the ionosphere, namely the D region (which extends from 60-100 km in altitude) is the most impacted by such ionising sources. It is most prominently disturbed by solar flares for which enhanced X-ray emission from the Sun penetrates down to the D-region and dominates ionisation acting on N₂ and O₂. This increases the local electron density to extents large enough to cause immediate space weather effects including high-frequency radio blackouts. The impact on the ionosphere in response to ionising disturbances is generally known as a Sudden Ionospheric Disturbance (SID).

The response of the D-region to ionising disturbances can be remotely probed through measurements of very low frequency (VLF; 3-30 kHz) radio waves that propagate in the waveguide formed between the Earth's surface and the lower ionosphere. The D-region acts as a sharp reflective boundary to VLF radio frequencies, which can then propagate over very large distances around the globe. In undisturbed conditions, the propagation is stable in both amplitude and phase, however when there is increased ionisation the electron density and conductivity profile of the upper waveguide boundary changes, which results in VLF amplitude and phase variations in a measured VLF signal. Narrowband measurements of VLF signals generated from worldwide communication transmitters can hence be used to detect ionospheric disturbances. Such measurements are routinely used in solar flares studies and their impact on the ionosphere (Ref. [1]). In rare cases extra-solar transients can also be detected in VLF measurements. These includextremely luminous gamma-ray sources such as magnetar flares (Ref. [2]) and even the brightest of the fairly common cosmic gamma-ray bursts gamma-ray_bursts (GRBs) (Ref. [3]). RHESSI Nugget No. 3 described the RHESSI observations of this first magnetar detection. Up until now the GRB events are mainly identified in the D-region at night, when interference by the solar ionizing radiation does not swamp the signal.

Here we report a rare example of a massive gamma-ray burst (GRB) so bright that it produced a SID with a solar-flare sized signature on the dayside ionosphere as measured with VLF observations over northern Europe. These observations demonstrate that an extragalactic GRB (at Z = 0.151) can have a significant impact on the terrestrial atmosphere.

Detection of Massive Gamma-ray Burst 221009A in Ionospheric Measurements

On October 9, 2022, an extremely bright and long-duration GRB was detected by both the Swift/BAT and Fermi/GBM telescopes. It is now noted as the largest GRB ever detected, and has been reported in observations from many missions, including the Orbiter/STIX instrument.

We also detected a co-temporal ionospheric disturbance from the SuperSID monitor that is operated at DIAS Dunsink Observatory in Dublin, Ireland. Here we used measurements of 23.5 kHz that are transmitted from a submarine-communications transmitter located in Germany (station ID: DHO38). Observations at this frequency are hence probing the region of the ionosphere between Ireland and Germany. The VLF measurements are shown in Figure 1(a) over the day, with the grey shaded regions marking the nighttime. The GRB impact can clearly be identified between the vertical dashed lines in (a), and is highlighted further in (b). The X-ray and gamma-ray observations from the STIX instrument on Solar Orbiter and Fermi/GBM are shown in (c ) and (d) respectively. The timing comparisons between the GRB high-energy signatures and the VLF response clearly indicates that the VLF peak is a result of the incident GRB fluxes. The VLF amplitude increase from the pre-GRB amplitude levels is 3.4 dB, which is a large disturbance. To put this in perspective, based on statistical studies of solar flare effects this amplitude increase is equivalent to a GOES C3-M1.0 X-ray flare!

In terms of timings, we find a time-delay between the impulsive GRB ionisation as detected with STIX and GBM and the ionospheric response of about 55 s, shorter than the typical 2-3 minutes often found in solar flare ionisation. This delay is due to the photoionization-recombination balances in the D-region ionosphere. In terms of a recovery, following the short impulsive burst of ionisation, the ionosphere takes a long time to recover which can be seen in the decay profile in (b), for which the disturbance does not reach pre-GRB levels until almost 30 minutes after the impulsive peak.

Conclusions

The observations presented here show that the extremely large GRB 221009A produced significant impacts on the ionisation of the lower ionosphere of our planet. From our understanding, this is the strongest effect ever produced on the ionosphere in response to ionising radiation from a GRB and adds to the small handful of cases reported before. The fact that this GRB was readily detected in the dayside ionosphere also has several interesting scientific implications - the short impulsive duration of the GRB acts as a delta-function input of ionisation into the ionosphere, and allows us to to perform studies of the ionising response on short timescales and investigate the recombination recovery following the input. This is something we rarely have the opportunity to do in solar flare studies given the comparatively long decay time of such events, as it is extremely challenging to disentangle the recombination timescales with an ongoing ionisation.

This work has been described in Ref. [4]. (Editor's note: Event, paper, and Nugget all appeared within one month, perhaps a record of some sort.)

References

[1] "Solar Flare Effects on the Earth's Lower Ionosphere"

[2] "Ionization of the lower ionosphere by {\ensuremath{\gamma}}-rays from a Magnetar: Detection of a low energy (3-10 keV) component"

[3] "Observation of an ionospheric disturbance caused by a gamma-ray burst"

[4] "A Significant Sudden Ionospheric Disturbance Associated with Gamma-Ray Burst GRB 221009A"