A Non-linear Reaction of the Ionosphere and Thermosphere to Solar Cycle EUV Variations

Transcription

1 A Non-linear Reaction of the Ionosphere and Thermosphere to Solar Cycle EUV Variations Andrei Mikhailov and Loredana Perrone Institute of Terrestrial Magnetism Ionosphere and Radio Wave Propagation (IZMIRAN), Russia Istituto Nazionale di Geofisica e Vulcanologia, Italy

2 The Problem Formulation 1. Solar EUV flux ionizing F2-region varies by time in solar cycle, while daytime midlatitude NmF2 varies by 5-6 times in and by times in. 2. Practically linear NmF2 increase in, but a saturation effect in This is a well-known feature of the F2-layer Solar Cycle variations

3 and median NmF2 Solar Cycle Variations NmF2x10 5 cm Boulder (Jan and Jul, 12 LT) NmF2x10 5 cm Juliusruh (Jan and Jul, 12 LT) NmF2x10 5 cm FS 10.7 Sverdlovsk (Jan and Jul, 12 LT) NmF2x10 5 cm FS 10.7 Irkutsk (Jan and Jul, 12 LT) FS FS 10.7

4 To understand the reason of these variations accurate ionospheric observations should be used and main Aeronomic Parameters: 1. Ionizing Solar EUV radiation 2. Neutral composition ([O],[O 2 ],[N 2 ]) and temperature T(h) 3. Vertical plasma drift should be specified for the conditions in question

5 Only Incoherent Scatter Observations can be used for such analysis NmF2x10 5 cm Juliusruh (Jan and Jul,12 LT) Millstone Hill ISR FS 10.7.

6 A comparison of EUV models with SOHO observations EUV Flux x (ph cm -2 s -1 ) ( )Å SOHO observations EUVAC model Nusinov model (10-500)Å EUV Model by A. Nusinov (1984) can be used for our aeronomic calculations EUV Flux x (ph cm -2 s -1 ) SOHO observations EUVAC model Nusinov model Months

7 A Self-Consistent Method for Modeling Ne(h) in the F2-region Using ISR Observations I N P U T Routine ISR Observations N e (h) T e (h) T i (h) V i (h) F2-layer Theoretical Model O U T P U T A Self-Consistent Set of Thermosphere Parameters T n = f(t ex, T 120, S) [O], [O 2 ], [N 2 ] W(h) V nx Ion Composition

8 Observed (ISR) and Calculated Ne(h) Profiles for and under Solar Maximum and Minimum Jan 14, 1990 Height, km Jan 09, LogNmF2, cm Jun 05, 1989 Height, km Aug 09, Log NmF2, cm -3

9 Calculated and Model Thermospheric Parameters at 300 km Tex, K Log[O 2 ], cm Log[O], cm Log[N 2 ], cm (F FA)/ (F FA)/2 NRLMSISE-00 model by Picone et al.

10 The retrieved aeronomic parameters controlling the Ne(h) distribution do not bear the shortcomings of thermosphere empirical models. They constitute a consistent set of basic parameters which can be used for quantitative estimates. This is the basic difference from other similar approaches

11 For mid-latitude daytime F2-layer the well-known expressions by Rishbeth and Barron (1960) can be used. NmF 2 2 = 0.75qm / βm βm = 0.6Dm sin I / H 2 where all parameters are given at the hmf2 height q m O + ion production rate D m ambipolar diffusion coefficient β m linear loss coefficient for O + ions H atomic oxygen scale height I magnetic inclination In fact the above expression for NmF2 and hmf2 reflect the idea of isobaric F2-layer by Rishbeth and Edwards (1989) the F2-layer follows the level of P=const in its variations

12 Calculated aeronomic parameters at the hmf2 height for and days under Solar Minimum and Solar Maximum lgnmf2 Date hmf2, km T ex K lg[o] m cm -3 lg[o 2 ] m cm -3 lg[n 2 ] m cm -3 γ 1 x10-13 cm 3 s -1 γ 2 x10-12 cm 3 s -1 q m x10 2 cm -3 s -1 β m x10-4 s -1 q m /β m x10 6 cm -3 W m s This set of aeronomic parameters enables us to make all quantitative estimates

13 Seasonal/Solar Cycle Variations of q m /β m ~NmF2 The q m /β m variations are: 5.91 time for and 2.64 times for. Ion production rate q m increases: by 2.29 times in and by 2.63 times in Loss coefficient β m decreases: by 2.58 times in and by 1.0 times in! when we pass from Solar Minimum to Solar Maximum

14 That is loss coefficient β does not practically change at the hmf2 height in and the NmF2 increase is totally due to q m increase. In the contributions of q m and β m to the NmF2 increase are comparable.

15 Under Δβm 1 the leading role in forming the Saturation Effect belongs to [O] m variations as NmF2 ~ q m /β m and q m ~ I EUV [O] m q(o + ) x 10 2 cm -3 s EUV flux x cm -2 s A two-component model by Nusinov log[o] m cm MSISE00 MSISE (F FA)/2 Notice - there is no saturation effect in EUV variations with solar activity as it is widely accepted to think (F FA)/2

16 Why Δβm is small in? [N 2 ] decrease at hmf2 height by 2.27 times 1.15 times γ (O + + N 2 ) increase at hmf2 height by 1.08 times 1.72 times That is in summer the γ (O + + N 2 ) increase overcompensates the [N 2 ] decrease While in the situation is quite different

17 The main difference between and is in Temperature variation range when we pass from Solar Minimum to Solar Maximum 1.8 Rate coefficient (10-12 cm 3 s -1 ) O + + N2 = NO + + N Rate coefficient by Hierl et al., Temperature, K

18 Seasonal Thermosphere Circulation Leading to the [O]/[N 2 ] Decrease This is the first step in the chain of processes leading to the NmF2 saturation effect Auroral zone Solar +Auroral Heating driven Seasonal (-to-) Vnx [O] increase [O] decrease Auroral zone

19 The Chain of Processes Leading to the Saturation Effect O/N 2 NmF2 summer decrease due to Tn increase, upwelling with summerto-winter hemisphere transfer decrease as NmF2 ~ q m /β m Te increase due to a decrease of electrons cooling Tn increase Tv γ (O + + N 2 ) β increase increase increase But this an avalanche type process stops in some steps at Te 2600K in the F2-layer under solar maximum due to: Confined solar EUV energy; Confined plasmaspheric energy reservoir; Cooling effect of winter hemisphere; Thermal conductivity Cooling in collisions with ions and neutrals NmF2 decrease as NmF2 ~ q m /β m

20 / difference in NmF2 variations is mainly due to different temperatures and a corresponding decrease in thermospheric species ([O], [N 2 ], [O 2 ]) at the hmf2 height (P = nkt constant) The leading role belongs to Temperature.

21 Conclusions 1. The observed NmF2 increase in Solar cycle is due to two reasons: one is EUV increase by a factor of 2, the other reason is due to [O] and β variations at the hmf2 height. 2. The difference between and in the course of Solar cycle is in temperature: T< 1200 K in and T >1200 K in. This results in different γ 1 (O + +N 2 ) and larger hmf2 in.

22 Conclusions 3. decrease in [O] and small variation in β = γ 1 [N 2 ]+γ 2 [O 2 ] at hmf2 under high solar activity results in the saturation effect in NmF2 under solar maximum. 4. The saturation effect in NmF2 results from a long chain of non-linear processes: O/N 2 NmF2 Te (Tn, Tv) γ 1 β NmF2 O/N 2 due to upwelling and so on.