the grapes-3 cosmic ray instrument: a space weather tool · 2018. 8. 3. · the grapes-3 cosmic ray...

IntroductionOverview

Forbush decreasesForbush Decrease (FD) precursors

FD precursors

The GRAPES-3 cosmic ray instrument: a space

weather tool

Prasad Subramanian

Indian Institute of Science Education and Research (IISER), Punehttp://www.iiserpune.ac.in

Subramanian FD precursors



FD precursors

Introduction

The GRAPES-3 cosmic ray instrument at Ooty (PI: Sunil

Gupta) is a versatile cosmic ray instrument operated by theTIFR




FD precursors

Introduction



It builds upon the long tradition of TIFR in cosmic rayresearch, and addresses science ranging from cosmic rayacceleration/knee studies and cosmic ray composition todetecting radioactivity in rain water




FD precursors

Introduction




The large (560 m2) muon detector makes it among the most

sensitive detectors in the world, capable of detecting changesin cosmic ray proton intensity of ∼ 0.1%




FD precursors

Introduction




The large (560 m2) muon detector makes it among the most

sensitive detectors in the world, capable of detecting changesin cosmic ray proton intensity of ∼ 0.1%

This makes it a sensitive space weather instrument




FD precursors

Overview

It has been recognized for a while (≈ 1930) that solartransients cause changes in the galactic cosmic ray intensitydetected at the Earth




FD precursors

Overview


Specifically, magnetic clouds/near-Earth CME manifestationsand their associated shocks are known to cause Forbushdecreases in the galactic cosmic ray intensity.




FD precursors

Overview


Specifically, magnetic clouds/near-Earth CME manifestationsand their associated shocks are known to cause Forbushdecreases in the galactic cosmic ray intensity.

More interestingly, precursors to Forbush decreases in galacticcosmic rays at the Earth provide advance information aboutparameters of the CME-associated shock, and hence theimpending geomagnetic storm




FD precursors

FDs are due to near-Earth CME/shock

Shock (magnetic “umbrella” against cosmic rays) + CME (lowdensity cavity)




FD precursors

Forbush decrease: GRAPES-3 data

Decrease in galactic cosmic ray intensity@Earth (due to largeICME/shock) on Oct 29 2003: data from GRAPES-3, Ooty




FD precursors

GRAPES-3 muon detector directional bins

GRAPES-3 can observe in 9 directions/rigidities/energies




FD precursors

Forbush Decrease studies with GRAPES-3

Conventional wisdom has it that the shock (propagating,diffusive barrier) and the CME (magnetic “bottle”) are bothresponsible for the observed Forbush decrease




FD precursors



At GRAPES-3 energies (≈ 14 – 42 GeV) we have shown(Subramanian et al 2009; Arun Babu, PhD thesis) that therelatively empty magnetic bottle (the CME) is the primarycause of the Forbush decrease.




FD precursors



At GRAPES-3 energies (≈ 14 – 42 GeV) we have shown(Subramanian et al 2009; Arun Babu, PhD thesis) that therelatively empty magnetic bottle (the CME) is the primarycause of the Forbush decrease.

We have derived tight constraints on the turbulence level(amplitude) in the vicinity of the CME (sheath region),especially with multi-rigidity data (Arun Babu, PhD thesis).We are also elucidating the role of cross-field diffusion in thepresence of MHD turbulence.




FD precursors

Forbush decrease due to magnetic bottle

CME starts out almost empty (of cosmic rays). Cosmic rays(cross-field) diffuse into it, but its still a depleted (cosmic ray)cavity when it reaches the Earth → Forbush decrease




FD precursors

FD precursors: summary

We now propose to work on the precursors to these FDs.




FD precursors



Use GRAPES-3 data to identify precursor decreases




FD precursors



Use GRAPES-3 data to identify precursor decreases

Theoretical understanding of FD precursors in terms of losscone decreases

A thorough understanding of FD precursors will result inpredictive capability (∼ 3 - 4 hours) wrt the strengthof/turbulence level near the shock that is due to hit theEarth’s magnetosphere




FD precursors

FD precursors: general understanding

The B field compression near the shock is like a magnetic mirror.Since v⊥ ↑ with B ↑, v‖ can → 0 at the mirror point for energy tobe conserved.




FD precursors

If v‖/v⊥ ≫ 1 (low pitch angle), the mirror effect is minimal, andthese particles can “leak” through the mirror. This “loss cone” ofparticles can propagate (at speeds ≈ c) ahead of the shock,forming a FD precursor.Cartoon: Nelson Schuch, Brazil




FD precursors

FD precursors: physics

Cosmic ray particles undergo resonant pitch angle scatteringwith turbulent irregularities near the shock. This alters theloss cone/precursor characteristics (Ruffolo et al 1999; 2003)

∂F

∂t= −

∂

∂z

[

µ v+

(

1−µ2 v2

c2

)

u

]

F+∂

∂µ

[

φ

2

∂

∂µ

(

1−µu v

c2

)

F

]




FD precursors



∂F

∂t= −

∂

∂z

[

µ v+

(

1−µ2 v2

c2

)

u

]

F+∂

∂µ

[

φ

2

∂

∂µ

(

1−µu v

c2

)

F

]

F : particle distribution function, z : distance from shock, µ:(cosine) pitch angle. So we have pitch angle diffusion due toresonance with turbulent eddies (embodied in the term φ:crucial!)




FD precursors



∂F

∂t= −

∂

∂z

[

µ v+

(

1−µ2 v2

c2

)

u

]

F+∂

∂µ

[

φ

2

∂

∂µ

(

1−µu v

c2

)

F

]

F : particle distribution function, z : distance from shock, µ:(cosine) pitch angle. So we have pitch angle diffusion due toresonance with turbulent eddies (embodied in the term φ:crucial!)

Characteristics of loss cone precursors will yield information onφ → turbulence amplitude, strength of shock




FD precursors

FD precursors: lead times

Net effect 1: Loss cone precursors for higher energy cosmicrays generally precede the FD by a longer time (complication:it also depends upon the geometry of the large-scale magneticfield/Parker spiral connecting the observer with the shock)




FD precursors

FD precursors: lead times

Net effect 1: Loss cone precursors for higher energy cosmicrays generally precede the FD by a longer time (complication:it also depends upon the geometry of the large-scale magneticfield/Parker spiral connecting the observer with the shock)

Net effect 2: Precursor lead time can also provide informationabout turbulence level near the shock/strength of shock(important!)




FD precursors

FD precursors: data analysis

There has been some work in the past on FD precursors(Nagashima et al) and a flurry of recent work from the globalmuon detector network (e.g., Munakata et al 2000; Fushishitaet al 2010; Rockenbach et al 2011)




FD precursors



Precursor decreases are generally rather weak; the largecollecting area/unprecedented sensitivity and directionalresolution of the GRAPES-3 muon telescope offers a distinctadvantage.




FD precursors




A first step would be to come up with a robust statisticalidentification of precusors in the existing GRAPES-3 Forbushdecrease database. Non-trivial, for the search involvesdifferent rigidities/energies and directions




FD precursors




A first step would be to come up with a robust statisticalidentification of precusors in the existing GRAPES-3 Forbushdecrease database. Non-trivial, for the search involvesdifferent rigidities/energies and directions

This could enable prediction of impending shock strengths




FD precursors

Conclusion: FD precursors with GRAPES-3

Forbush decrease precursor data from GRAPES-3 thus offerssignificant promise for diagnostics of turbulence levels nearshocks/shock strengths with lead times of at least a few hours.

The (very) interesting physics apart, it could serve as a effectivespace weather tool.

Thank you for your attention!


the grapes-3 cosmic ray instrument: a space weather tool · 2018. 8. 3. · the grapes-3 cosmic ray...

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