[Paper Review] Forbush Decreases during the DeepMin and MiniMax of Solar Cycle 24

Dimitra Lingri, H. Mavromichalaki+5 more|arXiv (Cornell University)|Dec 28, 2016

Solar and Space Plasma Dynamics3 citations

TL;DR

This study analyzes 749 Forbush decreases (FDs) in galactic cosmic ray intensity from January 2008 to December 2014 during Solar Cycle 24's deep minimum and mini-maximum phases, using neutron monitor data and the IZMIRAN FD database. It identifies precursor signatures—particularly pre-decreases in cosmic ray intensity at specific asymptotic longitudes—offering a forecasting tool for FDs, with 7 out of 58 high-anisotropy events showing detectable precursors and 45% of FDs occurring in 2014 alone.

ABSTRACT

After a prolong and deep solar minimum at the end of solar cycle 23, the current cycle 24 is one of the lowest cycles. The two periods of deep minimum and mini-maximum of the cycle 24 are connected by a period of increasing solar activity. In this work, the Forbush decreases of cosmic ray intensity during the period from January 2008 to December 2014 are studied. A statistical analysis of 749 events using the IZMIRAN database of Forbush effects obtained by processing the data of the worldwide neutron monitor network using the global survey method is performed. A further study of the events that happened on the Sun and affected the interplanetary space, and finally provoked the decreases of the galactic cosmic rays near Earth is performed. A statistical analysis of the amplitude of the cosmic ray decreases with solar and geomagnetic parameters is carried out. The results will be useful for space weather studies and especially for Forbush decreases forecasting.

Motivation & Objective

• To investigate the statistical behavior of Forbush decreases (FDs) in cosmic ray intensity during the low-activity phase of Solar Cycle 24, including the deep minimum and mini-maximum.
• To examine the relationship between FD amplitudes and solar/interplanetary parameters such as CME speed, solar wind velocity, and geomagnetic indices (Dst).
• To identify and characterize precursor signals—specifically pre-decreases and pre-increases—in cosmic ray intensity that may enable early forecasting of FDs.
• To assess the effectiveness of the Ring of Stations (RS) method in detecting anomalous cosmic ray anisotropy prior to shock arrival, using high-resolution neutron monitor data.

Proposed method

• Utilized hourly, pressure-corrected cosmic ray intensity data from 50 global neutron monitors via the NMDB database.
• Applied the Global Survey Method (GSM) to calculate the first harmonic of cosmic ray anisotropy at 10 GV rigidity to quantify intensity variations.
• Extracted FD events from the IZMIRAN database, focusing on those with amplitude >2%, and selected 114 events with amplitude >2% and 15 with amplitude >5% for detailed analysis.
• Employed the Ring of Stations (RS) method to detect longitudinal precursor patterns, using anisotropy thresholds (Axy > 0.9%) and requiring pre-shock quiet interplanetary conditions.
• Correlated FD events with solar and interplanetary data, including CMEs (from SOHO/LASCO), solar flares (GOES), and IMF parameters from ACE/Wind and OMNI databases.
• Selected and analyzed the 17 March 2013 event as a case study, using asymptotic longitude-time diagrams and anisotropy profiles to visualize precursor behavior.

Experimental results

Research questions

• RQ1 What is the statistical distribution of Forbush decreases in cosmic ray intensity during the deep minimum and mini-maximum of Solar Cycle 24?
• RQ2 How do FD amplitudes correlate with solar wind speed, CME velocity, and geomagnetic indices such as Dst?
• RQ3 Can precursor signals—specifically pre-decreases and pre-increases in cosmic ray intensity—be reliably detected before FD onset?
• RQ4 To what extent do anomalous cosmic ray anisotropy levels (Axy > 0.9%) precede FD events, and how do they relate to interplanetary shock arrival?
• RQ5 What role do CMEs and associated shocks play in generating large-amplitude FDs, and how do their properties influence FD magnitude?

Key findings

• Of 749 FD events recorded from 2008 to 2014, only 114 had amplitudes >2%, with 90 occurring during the mini-max phase (2012–2014), and 45% of these (45 events) occurring in 2014 alone.
• No strong correlation was found between FD amplitude and solar wind or CME velocities, indicating that other factors such as magnetic structure or shock geometry may dominate amplitude variation.
• Only seven out of 58 FD events meeting the anisotropy criterion (Axy > 0.9%) exhibited detectable precursor signals, suggesting that such precursors are rare but potentially valuable for forecasting.
• The 17 March 2013 event showed a clear precursor: a pre-decrease of >2.25% in cosmic ray intensity over a narrow longitudinal range (0°–180°), preceding shock arrival by ~24 hours.
• The amplitude of the FD in the 17 March 2013 event was 4.6% at 10 GV, consistent with a strong interplanetary shock and associated Dst depression of -75 nT.
• Pre-decreases occurred in a narrow longitudinal zone, while pre-increases were observed in broader regions, supporting the idea of localized, directional modulation of cosmic rays prior to shock passage.