Characterization of the space environment at Mercury’ orbit

P. Diego, M. Laurenza, M.Storini, and S. Massetti

IAPS/INAF - Italy

• Space Climate study to point out environmental conditions

between 0.3 and 0.47 A.U. for BepiColombo mission.

Short–term and long–term solar variability are considered

Results are compared to OMNI data at 1 A.U.

• Mercury’s magnetosphere response to different SW inputs are

considered

• Suprathermal particles flux occurrences and fluxes are analyzed

Index

Solar Activity - Short Term Variability

• Corotating Structures; 27-days recurrent streams, (HSSs from Coronal Holes) ,Solar Magnetic Sectors

• Transient Solar Phenomena; Solar Flares, ICMEs (e.g. Magnetic Clouds (MC), Non Compressive Density Enhancements

(NCDE))

In order to identify occurrences of persistent conditions at Mercury’s orbit only the largest (duration > 12h) perturbations have been selected.

The durations of this short term variability span from hours to days.

Shorter time-scale fluctuations are not considered for our purpose.

Solar Activity Long Term Variability

• Solar Activity cycle; duration, amplitude and phases, Gnevyshev Gap

• Even and odd solar cycles;

The time scale of this variability spans from few months (cycle increase / decrease, G-Gap,…) up to 22 years (Hale cycle)

Data Used and Events Selection

Helios 1 and 2 hourly data for period from 1975 to 1980 between 0.3 and 0.47 A.U. (about 2x30 days/year)

Perturbation category individuated are:• HSSs from CH (e.g. Mavromichalaki et

al. 1988)• ICME from active regions. Among

them we selected:• Magnetic Clouds (MC – Burlaga

1981)• Non Compressive Density

Enhancements (NCDE – Gosling et al. 1977)

22 HSSs, 11MCs, and 27 NCDEs have been identified

Helios 1 data for April 1977.Orange selections indicate HSSs, grey a NCDE, and blue a Magnetic Cloud

Number of selected perturbations for each Helios, 1and 2, passage at Mercury’s orbit along with Solar Cycle n. 21 profile.Interplanetary Shock numbers (Volkmer and Neubauer, 1985) are superimposed (red markers).

Solar Perturbations occurrences are displayed in % of total observation time.

High percentage could depend on reduced time of observations.

Why do we separate classes of perturbations?Because the relative percentage of each class depends on long term solar activity variations such as phase, amplitude.• The phase of cycle; by definition is the temporal evolution of Sunspots Number • Odd numbered cycle features; enhanced transient activity during rise and maxima, higher

amplitude of maxima (Cliver et al. 1996)• Even numbered features; enhanced co-rotating solar structure during descending phases• The amplitude of solar cycle determines the ratio of active regions and CH at mid solar

latitudesWhile the first two are easily identified, the third one should be forecasted.

To forecast the solar cycle amplitude we could study the persistence of polar magnetic structures. In fact for some dynamo models [e.g. Dikpati et al., 2004;Choudhuri et al., 2008] the polar field is essential for the generation of sunspots of the subsequent cycle, although their persistence could lead to opposite results when predicting the sunspots maximum amplitude.

Polar CH lifetime and extension are inversely related to the strength of Sun's polar fields of the following activity cycle[e.g. Rowse and Roxburgh, 1983; Bravo and Stewart, 1994].

Diego et al. (JGR, 2009) performed an alghoritm to evaluate the CH persistence by analysinga century of corotating perturbations effect on geomagnetic activity.The Persistence index computed during descending phases shows a high anti-correlationwith the following solar activity cycle amplitude.

Global Sunspot’s Areas for solar cycle n. 24 has been predicted to be the lowest of the last 100 years!

Range of V is nearly constant from 0.3 to1 A.U.

Ratio between average values of Helios and OMNI data (H/O) is:

H/O(Vtot) = 1.0

H/O(VHSS) = 1.0

H/O(VMC) = 0.9

H/O(VNCDE) = 0.9

Transient Perturbations show lower values (200-280 km/s) not fount at 1 A.U.

Range of D is near 10 times higher at Mercury’s orbit

Ratio between average values of Helios and OMNI data (H/O) is:

H/O(Dtot) = 9.2

H/O(DHSS) = 10.2

H/O(DMC) = 11.6

H/O(DNCDE) = 8.8

Relative position of different classes of perturbation remains the same

Range of T is near 2 times higher at Mercury’s orbit

Ratios between averaged values of Helios and OMNI data (H/O) is:

H/O(Ttot) = 2.0

H/O(THSS) = 1.9

H/O(TMC) = 1.5

H/O(TNCDE) = 1.9

Relative position of different classes of perturbation remains the same

Range of B is nearly 4 times higher at Mercury’s orbit

Ratios between averaged values:

H/O(Btot) = 5.0

H/O(BHSS) = 6.3

H/O(BMC) = 3.6

H/O(BNCDE) = 4.8

In the inner heliosphere HSSs have IMF B values.comparable to transient’s ones.The radial gradient is greater of about a factor 2 for HSS with respect to transients (according to Burlaga’s (2001) global range of variability 3-6)

Range of Bx is nearly 6 times higher at Mercury’s orbit

Ratios between averaged values :For positive values:H/O(Bxtot) = 7.0H/O(BxHSS) = 8.1H/O(BxMC) = 5.4H/O(BxNCDE) = 6.1

For negative values:H/O(Bxtot) = 7.6H/O(BxHSS) = 9.9H/O(BxMC) = 5.4H/O(BxNCDE) = 7.0

Results in agreement with Parker.

The Bx of HSSs has the same behavior as B

Range of By is nearly 3 times higher at Mercury’s orbit

Ratios between Ranges:H/O(Bytot) = 2.9

H/O(ByHSS) = 4.1

H/O(ByMC) = 2.4

H/O(ByNCDE) = 3.2

Results in agreement with Parker.

Contributions on Alfvenic fluctuations during HSSs enlarge the range at Mercury’s orbit (Korth et al., 2011, Diego et al.2005)

Range of Bz is near 4 times higher at Mercury’s orbit

Ratios between Ranges:

H/O(Bztot) = 3.7

H/O(BzHSS) = 4.2

H/O(BzMC) = 3.1

H/O(BzNCDE) = 4.3

Results show radial decrease ≈ 1/r and a ratio Bz/By ≈ 0.6In agreement with Mariani et al. (1979)

B (nT) BxPos (nT) BxNeg (nT) By (nT) Bz (nT) D (cm-3) T (K) V (km/s)Parker Spiral (deg)

Total 33.7 23.8 -24.4 0.7 0.3 68.9 242800 417.6 -16.9

Total_std 9.9 10.8 9.8 13.6 10.7 47.5 162700 120.6 27.3

HSSs 35.3 26.6 -26.7 1.2 0.8 37.9 383258 541.8 -15.2

HSSs_std 7.6 8.2 8.9 11.6 8 15.9 163200 114 21.02

MCs 32.8 21.2 -19.9 -1.8 0.6 111.7 145023 337.6 -17.4

MCs_std 12.3 11.6 9.4 16.3 14.6 44.1 91030 49.1 35

NCDEs 30.5 19.4 -21.7 0.4 0.4 99.4 149800 338.3 -19.6

NCDEs_std 11.2 10.8 11.4 14.4 11.3 48.7 105300 61.7 32.8

Helios Data between 0.3 and 0.47 A.U.Average values

Average values of Total dataset are in agreement with previous works (e.g. Burlaga 2001 and reference therein).

B (nT) BxPos (nT) BxNeg (nT) By (nT)(*)

Bz (nT)(*) D (cm-3) T (K) V (km/s)

Parker Spiral (deg)

Total 5.0 7.0 7.6 3.5 -10.0 9.2 2.0 1.0 0.8

Total_std 3.2 5.4 4.7 2.9 3.7 8.5 1.6 1.2 0.6

HSSs 6.3 8.1 9.9 -1.2 -13.3 10.2 1.9 1.0 0.7

HSSs_std 5.1 4.8 5.9 4.1 4.2 9.4 1.6 1.3 0.5

MCs 3.6 5.4 5.4 -2.0 -12.0 11.6 1.5 0.9 0.5

MCs_std 3.2 4.5 4.1 2.4 3.1 7.0 1.0 0.7 0.7

NCDEs 4.8 6.1 7.0 -1.0 2.0 8.8 1.9 0.9 1.1

NCDEs_std 3.6 5.7 5.7 3.2 4.3 6.0 1.0 0.9 0.7

Ratios between Helios and OMNI.

(*) Average values of OMNI data close to zero

Magnetospheric Model – Toffoletto Hill modified by MassettiEmpirical–analytical magnetospheric model starting from the Toffoletto–Hill TH93Code (Toffoletto and Hill, 1989, 1993). The new model has been fine tuned to approximately match the Mariner10 data (flyby III), and to reproduce some of the key features of the self-consistent hybrid model of Kallio and Janhunen (2003).IMF BX component in the model contributes to depict a realistic IMF–magnetosphereReconnection geometry.Dipole values used is computed by Anderson et al, (2011) as 195 ± 10 nT/RM3

HSSs Positive

x

y

x

z

HSSs Negative

Sketch of the Mercury’s magnetosphere, computed for different values of IMF components , showing the spatial distribution of the field lines, which are: • open on the dayside (XGSM≥-1), (red)• open on the tail (XGSM<-1), (grey).• closed (blue).

MC Positive

x

y

x

z

x

y

x

z

MC negative

NCDE Positive

x

y

x

z

x

y

x

z

NCDE Negative

Intensity emission (kiloRayleigh) of the D2 line of Na (brightest sodium line). White profile evidences the planet position (dotted meridian are for the region not illuminated by the Sun). On the left a double spot probably due to IMF component sudden variations. On the right a single spots as in the case of defined IMF polarity. The input is a HSS the magnetic configuration could last for several days.

Courtesy of V. Mangano (IAPS-INAF)

Suprathermal Particles

SEP dependence on solar activity has been extensively investigated (e.g. Nymmik, 1999, Storini et al. 2008) and many computational model has been performed (see Vainio et al. for a review). Part of their database is summarized in figure above (Laurenza et al. 2007). Peak flux upper limit at 1 A.U. is close to 105 pfu on hourly averaged data.

28 April 1978 SEP event is one of identified magnetically connected events (Gardini et al. (2011) for a list)

Helios 1 and 2, and IMP 8 magnetically connected within 20°; separated of 0.7 A.U. in radial distance.

Proton fluxes recorded in the energy range 4−40 MeV by Helios 1, Helios 2,

IMP 8 spacecraft during 1974−1982 are used to analyze all SEP events for

which at least two spacecraft have their nominal magnetic footpoint within

20◦ in heliocentric longitude from each other.

Radial dependence ~ R

α = 2.13

r = 0.99

The upper limit of Peak Fluxes at

0.3 A.U. is ≈ 106 pfu

(for >10 MeV proton energy)

Summary

• Helios 1 and 2 data for 1975-1980 have been used to evaluate the space

environment between 0.3 and 0.47 A.U. in the most similar condition s for

BepiColombo mission.

• Statistical features for different classes of perturbations have been

compared between Mercury’s and Earth’s orbits

• Relative occurrence s of corotating and transient perturbations have been

studied in relation to long term solar variability and its predictability.

• Response of Mercury’s Magnetosphere for different SW inputs are

displayed to identify field lines connected to external environment

• Suprathermal particles fluxes data for two solar activity cycles have been

used to evaluate occurrence, peaks and fluences at Earth’s orbit.

• The same parameters (upper limit) can be inferred at Mercury’s orbit by

using the radial gradient computed with Helios and IMP8 data.