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RISER events

We have identified several events we want to study in the RISER project, summarised below, in table and visual forms.

These events have been selected as times where space weather has caused enhanced geomagnetic activity on Earth, characterised using the Kp index, and where interplanetary scintillation (IPS) observations are available.

Context for RISER event selection

Geomagnetic enhancements have been used as the basis for RISER event selection as they can have impacts on Earth and human technology (e.g. potentially causing power grid issues). They arise when Earth's magnetic field gets disturbed by the arrival of structures launched from the sun, notably coronal mass ejections (CMEs) and corotating interaction regions (CIRs).

Not all CMEs and CIRs cause significant geomagnetic disturbances: understanding which ones have the potential to cause problems is a key issue for space weather research.

Current observation sources and modelling approaches used operationally, based on observations of the sun and its environs, give some advance warning (typically 3-4 days) of the presence of Earthbound CMEs or CIRs. However there are long intervals on their passage to Earth where these structures are not monitored operationally. And it is only only when they reach satellites at the L1 Lagrangian point, 99% of the way to Earth, that their potential impact becomes clearer. This gives only about 1 hour of warning, insufficient for most practical purposes.

The RISER project is exploring whether using IPS observations to remote sense the undermonitored region between the sun and Earth can help matters. IPS observations may allow an improved monitoring of CME and CIR structures, and the overall solar wind they travel in, and may also allow forecasting of impacts before these structures reach L1.

Specifically, for the events here, RISER is seeking to explore how additional IPS observations from the LOFAR radio telescope can complement existing IPS measurements from the ISEE radio telescope, and let us better characterise CME and CIR structures, and forecast their impacts on Earth.

In order to translate any resulting research outcomes from RISER into space weather operations it is vital that any associated diagnostics can be provided accurately, robustly, and rapidly - far enough in advance to provide useful and actionable forecasts for affected sectors. To explore the potential for operational use, RISER will not only run the IPS modelling systems in near-real time, but will also seek to verify its performance, both on the historic events below (running the systems in "hindcast" mode), but also on any new events which occur during the near-real time runs.

We also want to understand how forecasts of CMEs and CIRs using IPS compare overall to existing operational ways of forecasting these structures. By benchmarking the forecast performance of IPS-based approaches, we can understand if this has potential as a complementary observation source, worthy of further investment to integrate into the operational space weather observing system.

Events

The table below records the various events of interest which have some level of IPS data availability, whether from the ISEE or LOFAR radio telescopes, or the spaceborne SMEI instrument.

For each event we record:

ID
an associated identifier
Kp
the maximum Kp occurring during the event
Event start
the first 3H interval for the event where Kp >= 5
Event start
the last 3H interval for the event where Kp >= 5
Type
whether catalogues suggest a CME is present, or if a CIR cause is presumed

ID Kp Event start Event end Type
R01 8 2024-03-23T21Z 2024-03-24T18Z CME
R02 6 2023-09-19T00Z 2023-09-25T00Z CME
R03 5 2019-08-31T09Z 2019-09-01T12Z CIR
R04 5 2020-09-24T06Z 2020-09-27T21Z CIR
R05 5 2018-11-04T21Z 2018-11-05T03Z CIR
R06 8 2017-09-08T00Z 2017-09-08T12Z CME
R07 8 2012-03-09T06Z 2012-03-09T06Z CME
R08 7 2015-03-17T12Z 2015-03-17T21Z CIR
R09 7 2011-08-05T18Z 2011-08-06T00Z CME
R10 7 2010-04-05T09Z 2010-04-05T09Z CME
R11 6 2011-05-28T06Z 2011-05-28T12Z CME
R12 6 2010-08-03T21Z 2010-08-04T18Z CME
R13 6 2011-09-26T15Z 2011-09-26T18Z CIR
R14 6 2008-09-04T03Z 2008-09-04T03Z CME
R15 6 2008-10-11T15Z 2008-10-11T15Z CIR
R16 6 2010-05-02T12Z 2010-05-02T12Z CIR
R17 6 2011-06-05T00Z 2011-06-05T00Z CME

Annual cycle view of the events

The image below gives a summary of the RISER events across the years.

Examining events by Earth-based calendar years is useful for considering terrestrial factors, such as the Russell-McPheron effect which enhances the geoeffectiveness of Earth-impacting solar structures at the spring and autumn equinoxes.

It is also useful for considering factors such as data availability: currently IPS measurements from the ISEE radio telescope in Japan are typically unavailable during the winter months from October to March, when the telescopes are often covered by snow.

RISER annual cycle

Annual cycle view of RISER events, contextualised against geomagnetic and solar conditions.Main subpanel: Kp as a function of day-of-year (x-axis) and date (y-axis). Labelled RISER events (downward triangles) and ICME arrivals at L1 (upward triangles) are overplotted. Right subpanel: solar cycle context showing 180-day smoothed sunspot number (x-axis) against date (y-axis, same date range as y-axis in main subpanel).

Carrington rotation view of the events

Another good way of reviewing the RISER events is via a Carrington rotation plot.

This is useful for considering the events on solar timescales - in this view, the events and associated Kp data are organised by the Carrington rotation rate of 27.2753 days, corresponding to the typical rotation rate of the sun near the equator (the sun rotates slower near the poles). This helps differentiate events associated with slowly-evolving solar conditions (such as CIRs), which show up as recurring "clouds" of activity, versus more transient events associated with rapid solar dynamics, such as CMEs, which show up as horizontal streaks.

RISER Carrington cycle
Carrington cycle view of RISER events, contextualised against geomagnetic and solar conditions. Main subpanel: Kp as a function of Carrington rotation phase (x-axis) and Carrington rotation (left y-axis; right y-axis is equivalent date). Labelled RISER events (downward triangles) and ICME arrivals at L1 (upward triangles) are overplotted. Transient horizontal enhancements of Kp mostly correspond to ICMEs; nebulous recurring enhancements correspond to high-speed streams. Right subpanel: solar cycle context for the events, showing sunspot number (x-axis) against date (y-axis, same date range as right y-axis in main subpanel). Daily and 180-day smoothed sunspot numbers are shown. Lower left: tilt angles of effective rotation periods for recurrent features seen in the main subpanel. Vertically-aligned features correspond to the Carrington rotation period; features tilted left/right have shorter/longer effective synodic rotation periods.

Citations

We gratefully acknowledge the use of the following observations and key Python-based software in creating the RISER event overview figures:

References